Humorist who won’t keep still? (3)

I believe the answer is:

wag

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'humorist' is the definition.
(wag is a kind of humorist)

I cannot understand how the rest of the clue works.

(Other definitions for wag that I've seen before include "Jocular person" , "Vibrate; jester" , "Move from side to side like dog's tail" , "Joker; move tail" , "Oscillate - wit" .)

I've seen this clue in the Evening Standard.

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