Not amateur (12)
I believe the answer is:
professional
'not amateur' is the definition.
('professional' is the opposite to 'amateur')
This is the entire 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" .)