Cat has a wound behind ear

I am not a vet, so ymmv.

The wound itself looks to already be healing so I'd leave it alone. Just keep an eye on it, and make sure it doesn't smell, or start leaking lots of puss, or the skin around it becoming inflamed, red and sore.

As for a vet trip, that would depend heavily on how common cat diseases are in your area. With that kind of cut (as long as it's not a bite) the transmission risk is very low, but keep an eye out for pain or behavioural changes.

I would not rule out something like ear mites or some ear infection. The cat gets annoyed by the itching and will shake their head and scratch/rub their ears; often leading to what you see on the photo you posted. Look inside the ear, if it's dirty then that is a definite sign of mites. Mites or some infection needs to be treated, as it will most likely not resolve itself. So when in doubt about the cause, I would definitely seek the advice of a vet.

As it looks now, you do not need to do anything - just let the healing process do its thing.

You need to keep an eye on the wound to see if it gets infected and if it does, you need to get a vet to clean and drain it.

The thing with cat claws is the wound might close before an infecton is healed, so it might spread under the skin creating an abscess (it can really make a mess when it finally drains if it does not get treated).

You might want to take a look at how the other cat managed to get inside and fix the point of entry to avoid this type of problem in the future :)