Number, any number, is required for game (6)
I believe the answer is:
tennis
'for game' is the definition.
(tennis is a game)
'number any number is required' is the wordplay.
'number' becomes 'ten' (ten is a kind of whole number).
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'required' is a charade indicator (letters next to each other).
'ten'+'n'+'is'='TENNIS'
(Other definitions for tennis that I've seen before include "Roger Federer's game - anyone for it?" , "Popular racket game - anyone for it?" , "Sport on a court" , "Game (where love is nought?)" , "Medieval, originally French, sport, codified by the British" .)