A hole in the skin?

I just found a cat (probably a stray) and just noticed that she has a wound near the top of her back leg. It's not bleeding but it really looks like a hole in her skin. I could probably stick a finger in there and touch her flesh.
Looks like an abscess that fell off (I had that happen to one of my rat before).

What should I do?
Should I try to clean it or leave it alone?

Are you keeping the cat or not? It really needs to see the vet to see if antibiotics are necessary.

If the cat will let you, I would wash the area with saline solution until you can get it to a vet.

I agree with the above-see if you can clean and get to a vet.

If the cat will let you close, get it to a vet.