1815 battle (8)
I believe the answer is:
waterloo
'1815 battle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for waterloo that I've seen before include "Final crushing defeat" , "Where Napoleon Bonaparte lost it" , "The last battle" , "6D 5A [TUBE STATION]" , "Battle; Abba hit" .)