Russian prison camp (5)
I believe the answer is:
'russian prison camp' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gulag that I've seen before include "place of detention" , "System of detention camps in former Soviet Union" , "Book by Solzhenitsyn 'The . . . . . Archipelago' about detention camps in USSR" , "USSR system of labour camps" , "Soviet detention camp" .)