Willow let wrestle at Waterloo (6)
I believe the answer is:
battle
'waterloo' is the definition.
(thesaurus)
'willow let wrestle' is the wordplay.
'willow' becomes 'bat' (willow can mean a cricket bat).
'wrestle' indicates anagramming the letters (I've seen this in other clues).
'let' with letters rearranged gives 'tle'.
'bat'+'tle'='BATTLE'
'at' acts as a link.
(Other definitions for battle that I've seen before include "Sustained fight" , "Sustained conflict" , "Struggle tenaciously" , "- - of 5 down" , "Combat, conflict" .)