Unreturnable serves (4)
I believe the answer is:
aces
'unreturnable serves' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aces that I've seen before include "never come back" , "Expert fliers" , "Playing cards with single pips on their faces" , "winning shots" , "They're high and low on cards" .)