Are you sure it's an extra claw? I can't think of anything that would cause that unless your cat has been subject to an extreme dose of some sort of radiation :p It would be like a human growing an extra finger nail when they're 50.
If I had a picture I could tell you what it is, here are my guesses though
a) You didn't realize cats have 5 claws
b) One of your cats claws 'split' and is making the appearance of an extra claw, but is really just 1 damaged claw
c) A 'dead' claw that got stuck in his pad