Unusually alert in court battle (8)
I believe the answer is:
waterloo
'battle' is the definition.
(I've seen this before)
'unusually alert in court' is the wordplay.
'unusually' indicates an anagram.
'in' indicates putting letters inside.
'court' becomes 'woo' ('woo' can be a synonym of 'court'**).
'alert' is an anagram of 'aterl'.
'aterl' placed within 'woo' is 'WATERLOO'.
(Other definitions for waterloo that I've seen before include "Losers may meet theirs" , "Final crushing defeat" , "Signal failure for French" , "Battle - station" , "Famous battle" .)