The game's half lost, love (3)
I believe the answer is:
loo
'the game's' is the definition.
(historic card game found in Jane Austen novels)
'half lost love' is the wordplay.
'half' suggests halving the letters.
'love' becomes 'o' (love means zero in tennis).
'lost' cut in half is 'lo'.
'lo'+'o'='LOO'
(Other definitions for loo that I've seen before include "WC - old card game" , "bog" , "In which one goes" , "Card-game" , "Rest room" .)