Really good service (3)
I believe the answer is:
ace
'good service' is the definition.
(a good serve in tennis)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ace that I've seen before include "Person who excels, a top card" , "The opponent can't return this serve" , "Excellent" , "One; A1" , "unit > of highest quality" .)