Group round pub's up for game (6)
I believe the answer is:
tennis
'for game' is the definition.
(tennis is a kind of game)
'group round pub's up' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'round' means one lot of letters goes inside another.
'pub' becomes 'inn' (an inn is a type of pub).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'set' going around 'inn' is 'sinnet'.
'sinnet' back-to-front is 'TENNIS'.
(Other definitions for tennis that I've seen before include "Ball-game" , "Racket-and-ball game" , "Henman's game" , "Popular racket game - anyone for it?" , "Sent in strangely to play the game" .)