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 within the letters.
However, I'm not sure how the hidden word 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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