Retreat when bubbles fill North Korea (4)

I believe the answer is:

nook

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'retreat' is the definition.
(nook is a kind of retreat)

I don't understand the rest of the clue.

(Other definitions for nook that I've seen before include "Comfy corner" , "Alcove" , "Corner - narrow recess" , "Secluded recess" , "Interior angle formed where two walls meet" .)

I've seen this clue in the Sydney Morning Herald.

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