Calm sentry that is becoming agitated (8)

I believe the answer is:

serenity

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'calm' is the definition.
(I know that serenity is a type of calm)

I don't understand how the rest of the clue works.

(Other definitions for serenity that I've seen before include "Calmness" , "Peacefulness" , "Peace, tranquillity" , "The absence of mental stress or anxiety" , "Peace and quiet" .)

I've seen this clue in The Globe and Mail.

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