Astound by contriving, without ado, to knock out (4)

I believe the answer is:

stun

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'astound' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for stun that I've seen before include "Daze, stupify" , "Daze, concuss" , "Confuse" , "what a cracker may do?" , "Nuts (anag)" .)

I've seen this clue in the Evening Standard.

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