It can be bad for the elbow, sport (6)
I believe the answer is:
tennis
'sport' is the definition.
(tennis is a sport)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tennis that I've seen before include "court contest" , "Henman's game" , "Racket sport" , "Racquet-game" , "Sport with grand slams?" .)