Prisoner experienced defeat (8)
I believe the answer is:
confound
'defeat' is the definition.
(thesaurus)
'prisoner experienced' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'experienced' becomes 'found' (finding is a kind of experiencing).
'con'+'found'='CONFOUND'
(Other definitions for confound that I've seen before include "Puzzle" , "Perplex" , "Bewilder" , "Surprise; defeat (plan)" , "Astonish and confuse" .)