After a drink, game for battle (8)
I believe the answer is:
waterloo
'for battle' is the definition.
(1815 battle)
'after a drink game' is the wordplay.
'after' means one lot of letters go next to another.
'a drink' becomes 'water' (water is an example).
'game' becomes 'loo' (historical card game).
'water' next to 'loo' is 'WATERLOO'.
(Other definitions for waterloo that I've seen before include "Meet one's - -" , "track from Sweden" , "Wellington's victory" , "Napoleonic end" , "Famous battle" .)