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 anagramming the letters.
'during' means one lot of letters goes inside another.
'court' becomes 'woo' (both can mean to romance someone).
'alert' with letters rearranged gives 'aterl'.
'aterl' going into '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" .)