Spoil game at home (4)
I believe the answer is:
ruin
'spoil' is the definition.
(ruining is a kind of spoiling)
'game at home' is the wordplay.
'game' becomes 'ru' (rugby union).
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
'ru'+'in'='RUIN'
(Other definitions for ruin that I've seen before include "Wrecked building" , "Devastate" , "Dilapidated building" , "state of collapse" , "Catastrophe" .)