For a professional (3)
I believe the answer is:
pro
This clue is a double definition.
'for' is the first definition.
(as in being 'pro' 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" .)