Paid player (3)
'paid player' is the definition.
(eg a professional footballer)
This is the entire clue.
(Other definitions for pro that I've seen before include "Golf expert" , "Athelete who plays for pay" , "Top player" , "Favouring the professional" , "Preposition meaning for, in favour of" .)
(Other definitions for professional that I've seen before include "Not amateur" , "Expert - one paid in sport etc" , "someone performing for a fee maybe" , "Deliberate infringement to prevent the other side scoring" , "Qualified" .)