Alert performing in court battle (8)
I believe the answer is:
waterloo
'battle' is the definition.
(1815 battle)
'alert performing in court' is the wordplay.
'performing' indicates an anagram (the letters move around or 'perform').
'in' means one lot of letters goes inside another.
'court' becomes 'woo' (both can mean to romance someone).
'alert' anagrammed gives 'aterl'.
'aterl' placed into 'woo' is 'WATERLOO'.
(Other definitions for waterloo that I've seen before include "Decisive defeat" , "Tracked site" , "Belgian battle site" , "London rail terminus" , "song" .)