Unreturnable serve (3)
I believe the answer is:
ace
'unreturnable serve' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ace that I've seen before include "Single unit on playing cards" , "Top card or champion" , "Player who excels" , "Person who excels, a top card" , "'wicked'!" .)