The clutch is probably slipping because it is oil soaked. Vey likely it can be cleaned with brake clean. You need to clean up the bell housing and pressure plate anyway.
As much labor as there is to get to the clutch, if in doubt, replace it. Clean it up and then try and make a determination. If there is even a shodow of doubt, replace it unless you don't mind going back in if it still slips.
I have suceeded in cleaning oil soaked clutches, but there were much easier to get to.
