Cushion not well held by prisoner (6)
I believe the answer is:
pillow
'cushion' is the definition.
(pillow is a kind of cushion)
'not well held by prisoner' is the wordplay.
'not well' becomes 'ill' ('ill' is the opposite to 'well').
'held by' is an insertion indicator.
'prisoner' becomes 'pow' (POW is an example).
'ill' put inside 'pow' is 'PILLOW'.
(Other definitions for pillow that I've seen before include "Soft place for the head" , "Cushion for sleep" , "Support for the head" , "Item of bedding" , "Sleeper's support" .)