Convict, detainee (8)
I believe the answer is:
prisoner
I believe this is a double definition.
'convict' is the first definition.
(I've seen this before)
'detainee' is the second definition.
(I've seen this before)
(Other definitions for prisoner that I've seen before include "Rep irons out the captive" , "Senior PR (anag) - one in detention" , "Person behind bars" , "One inside" , "Person held captive as by law" .)