Paintings found in prisoner's box (6)
I believe the answer is:
carton
'box' is the definition.
(carton is a kind of box)
'paintings found in prisoner's' is the wordplay.
'paintings' becomes 'art' (I've seen this before).
'found in' means one lot of letters goes inside another.
'prisoner' becomes 'con' (abbreviation for 'convict').
'art' placed within 'con' is 'CARTON'.
(Other definitions for carton that I've seen before include "Sydney -, Tale of Two Cities hero" , "Box made of cardboard" , "Dickensian hero" , "Small, light box or container" , "Flapped cardboard box" .)