How the Americans find duck soup (4)

I believe the answer is:

easy

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for easy that I've seen before include "for a piece of cake" , "careful!" , "Posing no difficulty" , "Like a walk in the park?" , "Simply achieved" .)

I've seen this clue in the Evening Standard.

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