In which fast service can be advantageous (6)
I believe the answer is:
tennis
'in which fast service can be advantageous' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for tennis that I've seen before include "Game played with racket and ball" , "Racquet-game" , "court contest" , "Game that depends on service" , "Game played on court" .)