Villain beat up a knight, gatecrashing banquet (10)
I believe the answer is:
malfeasant
'villain' is the definition.
(thesaurus)
'beat up a knight gatecrashing banquet' is the wordplay.
'beat up' becomes 'mal' (I can't explain this - if you can you should believe this answer much more).
'knight' becomes 'n' (chess).
'gatecrashing' is an insertion indicator.
'banquet' becomes 'feast' ('feast' can be a synonym of 'banquet').
'a'+'n'='an'
'an' going inside 'feast' is 'feasant'.
'mal'+'feasant'='MALFEASANT'
(Another definition for malfeasant that I've seen is " criminal".)