Amateur's opposite (3)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
pro
'amateur's opposite' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pro that I've seen before include "Skilled" , "Favouring the professional" , "Top player" , "Competent person" , "For; non-amateur" .)