They're sealed in secrecy (4)

I believe the answer is:

lips

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I'm afraid I can't explain this answer.

(Other definitions for lips that I've seen before include "Edges of the mouth" , "margins" , "Folds of tissue round the mouth" , "Fleshy folds on the face" , "Fleshy folds of tissue" .)

I've seen this clue in the Evening Standard.

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