After watching a feral cat in action, they are dangerous. They will fuck you up if you mess with them. I was amazedA fox taking out a fully grown and healthy cat is extremely rare. Most won't tangle with a fully developed cat, which is quite capable of taking care of itself with unclipped claws in a fight against a fox.
If the two clashed, this is the likely outcome. Lots of hissing, posturing, then going about their business after a minor skirmish.