A witty movement? (3)

I believe the answer is:

wag

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'movement?' is the definition.
(wag is a kind of movement)

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

(Other definitions for wag that I've seen before include "The wit to move the tail" , "Jocular person" , "Vibrate; jester" , "He must be joking" , "Brisk rear movement or funny fellow" .)

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