For a professional (3)
I believe the answer is:
pro
I believe this clue is a double definition.
'for' is the first definition.
(as in being 'pro' or in favour of something)
'a professional' is the second definition.
(abbreviation)
(Other definitions for pro that I've seen before include "Expert (abbr)" , "Favouring the professional" , "Golfer" , "For; non-amateur" , "Professional; advantage" .)