Opposite of anti (3)
I believe the answer is:
pro
'opposite of anti' is the definition.
('pro' is the opposite to 'anti')
This is the entire clue.
(Other definitions for pro that I've seen before include "Expert (abbr)" , "Favouring the professional" , "Preposition meaning for, in favour of" , "Competent person" , "In brief, he's no amateur" .)