Prisoner discovered puzzle (8)
I believe the answer is:
confound
'puzzle' is the definition.
('confound' can be a synonym of 'puzzle')
'prisoner discovered' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'discovered' becomes 'found' (finding is a kind of discovering).
'con'+'found'='CONFOUND'
(Other definitions for confound that I've seen before include "Mix up" , "Thwart - baffle" , "floor" , "Befuddle, confuse" , "Surprise; defeat (plan)" .)