Say nothing when incarcerated (4,2)

I believe the answer is:

shut up

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'say nothing' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for shut up that I've seen before include "Close your mouth!" , "Imprisoned" , "jail" , "Rude order to stop talking" , "Stop talking - close the shop" .)

I've seen this clue in the Evening Standard.

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