Extremely unusual silence besets camp (5)
I believe the answer is:
gulag
'camp' is the definition.
(I know that gulag is a type of POW camp)
'extremely unusual silence besets' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'silence' becomes 'gag' (gagging is a kind of silencing).
'besets' means one lot of letters goes inside another (beset can mean to surround).
'unusual' with its centre removed is 'ul'.
'ul' going within 'gag' is 'GULAG'.
(Other definitions for gulag that I've seen before include "Russian labour camp system" , "Soviet forced labour camp(s)" , "Former prison camp in the USSR" , "Soviet prison camps" , "Russian prison and labour camp organisation" .)