Alert criminal in court defeat (8)
I believe the answer is:
waterloo
'defeat' is the definition.
(I know that waterloo is a type of defeat)
'alert criminal in court' is the wordplay.
'criminal' indicates anagramming the letters.
'in' indicates putting letters inside.
'court' becomes 'woo' (both can mean to romance someone).
'alert' is an anagram of 'aterl'.
'aterl' inserted into 'woo' is 'WATERLOO'.
(Other definitions for waterloo that I've seen before include "Historical action" , "Napoleon's final defeat" , "Historic battle" , "Wellington's victory" , "Seminal European battle of 1815" .)