Possibly a diamond expert (3)
I believe the answer is:
ace
I believe this is a double definition.
'possibly a diamond' is the first definition.
(I've seen this before)
'expert' is the second definition.
(ace is a kind of expert)
(Other definitions for ace that I've seen before include "Expert (sl.)" , "One in a suit" , "Winning serve; hole in one" , "hotshot" , "1" .)