Silence about limits of unusual prison (5)
I believe the answer is:
gulag
'prison' is the definition.
(gulag is a kind of prison)
'silence about limits of unusual' is the wordplay.
'silence' becomes 'gag' (gagging is a kind of silencing).
'about' is an insertion indicator.
'limits of' suggests removing the centre.
'unusual' with its centre taken out is 'ul'.
'gag' going around 'ul' is 'GULAG'.
(Other definitions for gulag that I've seen before include "penal system" , "Soviet political prison system" , "USSR system of labour camps" , "Former prison camp in the USSR" , "Soviet camps for dissidents" .)