Penniless knight defeated (6)
I believe the answer is:
broken
'defeated' is the definition.
(thesaurus)
'penniless knight' is the wordplay.
'penniless' becomes 'broke' (I've seen this before).
'knight' becomes 'N' (chess abbreviation).
'broke'+'n'='BROKEN'
(Other definitions for broken that I've seen before include "Shattered, perhaps emotionally" , "Busted, damaged" , "Interrupted; inoperative" , "Fractured, damaged" , "Smashed, in pieces" .)