Has the Defiant beaten any enemy ship comparable to a 'Galaxy Class' starship in single combat?

Although it's a fairly speculative answer (since in-universe there's never been a direct conflict between the two ships) we can make some reasonable suppositions based on the relative performance versus ships from enemy races and the one instance in which the Defiant faced a reasonably armed Federation ship.

The Galaxy class starship is described as "the largest and most powerful starship of its time" whereas the Defiant-class is characterised as a "limited role weapons platform".

In a 'stand-up' fight, the Galaxy-class would be the clear and immediate winner. Not only does it have vastly more phaser banks, stronger shields and a much higher top speed (warp 9.9 versus 9.5) but it also carries a host of additional weapons such as spatial mines.

Defiant-Class :

Speed: Warp 9.5

Armament: 4 forward-facing phaser cannons, at least 3 phaser emitters, at least 4 forward torpedo (photon and quantum torpedo) launchers, at least 2 aft torpedo launchers

Defenses: Deflector shields, ablative armor

Versus

Galaxy Class:

Speed: Warp 9.9 (Automatic engine shutdown after 10 minutes)

Armament: 12/14 phaser arrays; 2 torpedo launchers; 250 photon torpedoes; antimatter mines

Defenses: Deflector shields

The USS Valiant went up against a Cardassian Galor class vessel near the outbreak of the Dominion War. According to the Red Squad cadets who took over control, they fought to a stalemate. The Valiant finished repairs and then destroyed the Cardassian ship. Galor-class Cardassian vessels are comparable to Galaxy-class Federation vessels.

In the DS9 episode Shattered Mirror, a defiant-class ship engages a Klingon Neg'var class craft and forces it into retreat. I don't know for certain that the Neg'Var was better armed and shielded than a Galaxy-class ship, but it is possible (some might point out this was in the mirror universe, but even so the ships' specs probably aren't that different from their prime universe counterparts).

When it was first introduced in DS9 it was as powerful in weapons as any ship but in a small, multiple layer hull and other redundancy of main systems so that it can take a pounding. The power generator must be a major innovation. Anyway, Defiant taking a pounding is much cheaper than trashing a Galaxy class ship with lots of science lab tech and crew facilities, with a factor of a hundred less people endangered. So while both ships may have similar overall capability, the Defiant can be in a knock down fight much longer with fewer casualties and costly damage. Also it routinely made a bigger difference than any other ship in the Dominion War against all enemies. It wasn't just its own firepower but also (with Sisko as captain, especially) its ability to coordinate groups, serve as command ship, that made it a great warship.

According to some ultra-Trekaphiles, the Defiant (captained by Worf) was downgraded in the big Borg battle in movie Star Trek First Contact, made to show lots of damage and needing rescuing by Enterprise to maintain the supremacy of Galaxy class in cannon and keep Enterprise the most interesting of ships (though Voyager with super armor of last episodes seems like toughest ever but that was timey-wimey and not referred to again).

Bit difficult to answer.

1. The Defiant is much smaller that the Galaxy so it necessarily means it is of the less survivable design (less hull means vital systems easier to damage). It is, in turn, increased with ablative armor (again: necessity for a smaller ship), but there is only so much you can do in and on a limited hull. Design is for a warship, while Galaxy is a versatile vessel, with emphasis on diplomatic and research missions (including long-range). On the other hand, The Defiant's description as being "first pure warship design" is very telling. Which means that by definition Defiant is designed to be expendable, cheap to build (comparably) and not a stinging loss of life when (not if) lost in combat. Another point - it's officially classified as an escort vessel, but from all the conversations no one really knows how to classify it. Gul Dukat describes it as a "one of the most heavily armed warships in the quadrant", so while it should be treated as an exaggeration, not sure how to downgrade it and where to stop.
2. Defiant was a clear winner in action vs USS Lakota (there may be some quibbling if it was a clear win for some, but there it is - Lakota was disabled). This was done with crew unfamiliar with the ship and almost certainly not trained to even lowest SF standard, so this could be counted as a double-win. Also this was a fight to disable, so necessarily Defiant pulled some punches. That could be counted as another half of a win.
3. If Galaxy-class can find itself in trouble with two Galor-class, then the Defiant being easy winner (according to Tom Riker) against three Keldon-class (which are upgraded Galors), then it stands to reason that Defiant would be also a winner against a Galaxy-class.
4. While Galaxy is much bigger and more powerful, Defiant is more maneuverable and has much greater punch packed into pulse phasers. Lakota scored nearly 100% hits, while Defiant maybe 40% (and pulse phasers maybe 30%), so there is something to be wished for in the accuracy department, but definitely not in the power of the weapons.

My opinion is that defeat of Lakota, being refitted Excelsior-class (which was a Starfleet flagship of two generations ago), is the closest to Galaxy (of course Excelsior is a very obsolete design by the time of the Defiant). However, taking into account crew inefficiency as described earlier it would count as a sort-of-answer. The quality of the crew is apparently a multiplier to hardware capabilities (and it's proven correct time and time again), and if we plug my improvised multiplication of 2.5 into the equation I believe that 2.5 Excelsiors is greater than one Galaxy.

So I would conclude that Defiant was put up against the ship that could compare to Galaxy-class, and I'm pretty confident it would win in one-on-one combat.

Technically? As with all things Trek, we have some inconsistency from the fact that the writers write things to be as good as they need to be for the story to work out as planned. In some cases, you see Defiant class thrashing Galor-class vessels and being said to be equivalent of three Keldon classes. In other cases, like the USS Valiant, we see one Galor-class ship just about draw with the defiant class, only for the Defiant-class to be repaired quicker. I would suppose how you would measure it. Straight up firepower, durability, and ability to act as a strategic flagship (or catch/escape in retreat at warp) I would lay money on a Galaxy-class. However, If we take into account that a Galaxy-class simply WON'T take on a Defiant to the end (not wanting to lose the civilians live aboard, more expensive to repair, more valuable as a strategic asset, etc) then in a more realistic setting, the Galaxy would simply retreat once it was clear that battle would be costly and it would safely outpace the Defiant. Additionally, my brother and I did a test before I posted. We played ST Legacy, him three matches as a Galaxy and me a Defiant, Where I won 2-1 matches, but all were very close matches. Then, to account for player (crew) ability, we switched, and that was 3-0, all in favour of the Galaxy. Those matches were not as close. Not sure how reliable you all consider that test, but it is there for what it is worth.

It isn't clear, but we do have circumstantial evidence that the Defiant could stand a reasonable chance against a Galaxy class ship.

1 - When battling the Excelsior class U.S.S. Lokota in Paradise Lost, the Defiant was able to come out on top, despite trying to avoid serious damage.

2 - During the rescue of the Cardassian high council in the Way of the Warrior, the Defiant was able to drop its shields while being attacked by a Vor'cha class Klingon battlecruiser. The Defiant did use the tractor beam to reduce the effectiveness of the attack, but it still had its shields down and had dealt with two Birds if Prey first.

3 - The mirror Defiant was able to take out the mirror equivalent to the Neg'var, the Klingon's most powerful ship. They had some help from Rom which caused a power failure, but the Defiant was still putting up a strong fight.

4 - In numerous battles, the Defiant has destroyed Cardassian Galor class warships and Dominion battle cruisers.

5 - It took a heavy beating from the Borg, but was still in very good condition considering. Most of the repairs were able to be completed in a few days, considering the bridge showed no signs of the battle in In the Cards, which occurred a few days after First Contact.