Prisoner of the Gulag (3)

I believe the answer is:

lag

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'prisoner' is the definition.
(slang term)

'the gulag' is the wordplay.
'LAG' is hidden in amongst the letters.
But, I am unsure how this is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for lag that I've seen before include "Insulate (pipes)" , "Hang back" , "Get behind; convict" , "Cover pipes" , "Prisoner" .)

I've seen this clue in the Evening Standard.

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