Perturbed when one had brandished at (6)

I believe the answer is:

shaken

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'brandished at' is the definition.
'shaken' can be an answer for 'brandished' (I've seen this before). I am not sure about the 'at' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for shaken that I've seen before include "Having been shocked or disturbed" , "James Bond liked his martinis this way, not stirred" , "Disturbed as martini usually is" , "Shocked; astonished" , "Astonished; vibrated" .)

I've seen this clue in the Evening Standard.

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