Offence beside goal reflected game (6)
I believe the answer is:
tennis
'game' is the definition.
(tennis is a kind of game)
'offence beside goal reflected' is the wordplay.
'offence' becomes 'sin' (I've seen this before).
'beside' says to put letters next to each other (I've seen this in other clues).
'goal' becomes 'net' (net is a kind of goal).
'reflected' shows that the letters should be reversed in order.
'sin'+'net'='sinnet'
'sinnet' reversed gives 'TENNIS'.
(Other definitions for tennis that I've seen before include "Racket-and-ball game" , "Ball game with a net" , "Game for two or four" , "Game played with racket and ball" , "Racket sport" .)