Alert goes off during court defeat (8)
I believe the answer is:
waterloo
'defeat' is the definition.
(waterloo is a kind of defeat)
'alert goes off during court' is the wordplay.
'goes off' indicates an anagram.
'during' indicates putting letters inside.
'court' becomes 'woo' (both can mean to romance someone).
'alert' anagrammed gives 'aterl'.
'aterl' inserted within 'woo' is 'WATERLOO'.
(Other definitions for waterloo that I've seen before include "Where Napoleon met his final defeat" , "Ultimate defeat - London station" , "Napoleonic end" , "Wellington's victory" , "railway terminal" .)