Can I saw off broken freehub

Cutting might work, but there could be follow-on problems so its very last-resort. If you go this way, cut parallel to the cogs, between the middle two. So a 10 speed you'd cut between 5 and 6. This should remove any lip on the bolt, allowing the remains of the body to slide off. You'll then need to use grips to undo the half-bolt poking out.

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First I'd suggest you start with conventional removal.

Initially shove a 10mm hex driver in the hole where the axle goes. A short one might not engage, you may need a longer one. I've also seen bikes with 8mm or 12mm hex fittings down inside the hub, so try different sizes till you feel a positive engagement.

You also require a lot of torque to break a freehub bolt loose. I have a 12mm hex driver that is 650mm long and copes well. If you use a hand-sized hex driver then it may need some taps from a hammer, mind out for breakage risk ! You can also secure the hex tool in a vise pointing up, and use your hands on the rim for torque/rotation.

Every freehub bolt I've undone was normal right-hand thread.

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Update - your photo clearly shows a 12 pointed socket in the middle of the freehub. This will fit a 6 sided hex driver, most likely 10mm but it might be 11mm or remote chance of being some weird imperial fractional size like 25/64".

Buy or borrow a 10mm hed driver tool something like this :

To show the end you need:

Not a silly little bit for a screwdriver handle. Then use the entire rim as a grip and unthread the bolt that is in the middle of your freehub.

You can do this, without cutting - and remember if it goes badly, the freehub was dead anyway.