Keep quiet about extremely unconventional camp system (5)
I believe the answer is:
gulag
'camp system' is the definition.
'gulag' can be an answer for 'camp' (I've seen this before). I'm unsure of the 'system' bit.
'keep quiet about extremely unconventional' is the wordplay.
'keep quiet' becomes 'gag' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'unconventional' with its centre removed is 'ul'.
'gag' placed around 'ul' is 'GULAG'.
Can you help me to learn more?
(Other definitions for gulag that I've seen before include "Soviet forced labour camp(s)" , "where state's enemies were held" , "USSR system of labour camps" , "Labour camp in the Soviet Union" , "Russian prison and labour camp organisation" .)