Sent in to break up the game (6)
I believe the answer is:
tennis
'the game' is the definition.
(tennis is a kind of game)
'sent in to break up' is the wordplay.
'to break up' indicates an anagram.
'sent'+'in'='sentin'
'sentin' with letters rearranged gives 'TENNIS'.
(Other definitions for tennis that I've seen before include "Court-based sport" , "Game for two or four" , "Sent in strangely to play the game" , "court contest" , "event" .)