Specialist, ace (6)
I believe the answer is:
expert
I believe this clue is a double definition.
'specialist' is the first definition.
(a specialist is an expert in a field)
'ace' is the second definition.
(both can mean a master of something)
(Other definitions for expert that I've seen before include "One practised" , "Having specialised knowledge" , "At the right moment" , "One has specialist knowledge or ability" , "Authority" .)