Joker making wild assumption on internet forum (3)

I believe the answer is:

wag

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'joker' is the definition.
(I've seen this in another clue)

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" , "Wave to and fro, as a dog's tail" , "Joker; move tail" , "Jocular person" , "Oscillate - wit" .)

I've seen this clue in The Irish Times.

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