Fantastic tennis winner (3)
I believe the answer is:
ace
This clue is a double definition.
'fantastic' is the first definition.
(ace can informally mean very good)
'tennis winner' is the second definition.
(I've seen this before)
(Other definitions for ace that I've seen before include "winner in court" , "One or 11 in cards" , "Unreturnable serve" , "Excellent" , "Great" .)