Group round hotel up for game (6)
I believe the answer is:
tennis
'for game' is the definition.
(tennis is a kind of game)
'group round hotel up' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'round' indicates putting letters inside.
'hotel' becomes 'inn' (inn is a kind of hotel).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'set' enclosing 'inn' is 'sinnet'.
'sinnet' backwards is 'TENNIS'.
(Other definitions for tennis that I've seen before include "Game for two or four" , "Sport Dwight Davis gave trophy for" , "Real or lawn game" , "Ball sport" , "Table -, - elbow" .)