Labour camp joke contains acceptable line (5)
I believe the answer is:
gulag
'labour camp' is the definition.
(Soviet Union labour camp)
'joke contains acceptable line' is the wordplay.
'joke' becomes 'gag' (a gag is a joke or witticism).
'contains' is an insertion indicator.
'acceptable' becomes 'u' (as in U and non-U speech).
'line' becomes 'l' (used when specifying particular lines from a poem).
'u'+'l'='ul'
'gag' placed around 'ul' is 'GULAG'.
(Other definitions for gulag that I've seen before include "Soviet camps for dissidents" , "System of detention camps in former Soviet Union" , "Soviet camp system" , "Harsh labour camp" , "Former prison camp in the USSR" .)