Expert type of service (3)
I believe the answer is:
ace
I believe this clue is a double definition.
'expert' is the first definition.
(ace is a kind of expert)
'type of service' is the second definition.
(Other definitions for ace that I've seen before include "fighter pilot" , "Single unit on playing cards" , "Winning serve; hole in one" , "Top card or champion" , "Excellent" .)