Support one kind of prisoner -- it's bad being kept in (6)
I believe the answer is:
pillow
'support' is the definition.
(thesaurus)
'one kind of prisoner it's bad being kept in' is the wordplay.
'one kind of prisoner' becomes 'pow'.
'it's bad' becomes 'ill' (associated in meaning).
'being kept in' is an insertion indicator.
'pow' placed around 'ill' is 'PILLOW'.
(Other definitions for pillow that I've seen before include "Cushion for sleep" , "Item of bedding" , "Sleeper's cushion" , "Head supporter in bed" , "Support for the head" .)