Sorry prisoner gets dull (8)
I believe the answer is:
contrite
'sorry' is the definition.
(synonyms)
'prisoner gets dull' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'gets' is a charade indicator (letters next to each other).
'dull' becomes 'trite' (I've seen this before).
'con'+'trite'='CONTRITE'
(Other definitions for contrite that I've seen before include "Feeling sorrow for sins" , "Full of remorse" , "Repentant about nice trot" , "Wholly penitent" , "Repentant, remorseful" .)