Battle song (8)
I believe the answer is:
waterloo
This clue is a double definition.
'battle' is the first definition.
(1815 battle)
'song' is the second definition.
(ABBA song)
(Other definitions for waterloo that I've seen before include "Napoleon's 1815 defeat" , "Downfall of Napoleon" , "Signal failure for French" , "Where Napoleon met his final defeat" , "6D 5A [TUBE STATION]" .)