Sanctions mouthpiece for leak (3,4)

I believe the answer is:

let slip

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'leak' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for let slip that I've seen before include "Allow to fall or reveal a secret accidentally" , "Inadvertently reveal (something)" , "Allow to escape (information etc)" , "Fail to keep Secret" , "accidentally revealed" .)

I've seen this clue in The Guardian.

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