A repentant prisoner? Unoriginal (8)
I believe the answer is:
contrite
'a repentant' is the definition.
(I've seen this in another clue)
'prisoner? unoriginal' is the wordplay.
'prisoner?' becomes 'con' (abbreviation for 'convict').
'unoriginal' becomes 'trite' (similar in meaning).
'con'+'trite'='CONTRITE'
(Other definitions for contrite that I've seen before include "in a sorry state indeed" , "Feeling sorrow for sins" , "Suffering remorse" , "Sorry for something done" , "Penitent, feeling guilty" .)