Amateurs' opposites (4)
I believe the answer is:
pros
'amateurs' opposites' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pros that I've seen before include "Persons with knowledge and skill" , "Opposites of cons" , "Positive arguments" , "Arguments in favour" , "Sportsmen who are paid" .)