Not amateur (3)
'not amateur' is the definition.
('professional' is the opposite to 'amateur')
This is all the clue.
(Other definitions for professional that I've seen before include "Paid sportsman" , "someone performing for a fee maybe" , "Deliberate infringement to prevent the other side scoring" , "Qualified" , "Expert - one paid in sport etc" .)
'not amateur' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pro that I've seen before include "Golfer" , "Competent person" , "Favouring the professional" , "Professional; advantage" , "Preposition meaning for, in favour of" .)