Pro's opponent (3)
I believe the answer is:
con
'pro's opponent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for con that I've seen before include "Swindle; steer" , "Learn; swindle" , "Swindle or trick someone" , "Have a reason not to" , "politician who's in the right" .)