Sport isn’t about entertaining opposing players (6)
I believe the answer is:
tennis
'sport' is the definition.
(tennis is a sport)
'isn't about entertaining opposing players' is the wordplay.
'about' indicates an anagram.
'entertaining' means one lot of letters goes inside another (inviting in, as for dinner).
'opposing players' becomes 'en'.
'isnt' is an anagram of 'tnis'.
'tnis' placed around 'en' is 'TENNIS'.
(Other definitions for tennis that I've seen before include "Game (where love is nought?)" , "- elbow; - shoe" , "Ball game with a net" , "Sport on a court" , "Racquet-game" .)