Superior prisoner joins good camp (5)
I believe the answer is:
gulag
'camp' is the definition.
(I know that gulag is a type of POW camp)
'superior prisoner joins good' is the wordplay.
'superior' becomes 'U' (U can mean upper-class, as in U and non-U speech).
'prisoner' becomes 'lag' (slang term).
'joins' is a charade indicator (letters next to each other).
'good' becomes 'g' (abbreviation).
'u'+'lag'='ulag'
'ulag' after 'g' is 'GULAG'.
(Other definitions for gulag that I've seen before include "in restraint here?" , "Labour camp for political prisoners" , "Soviet forced labour camp(s)" , "Soviet camps for dissidents" , "Soviet prison camps" .)