A piece of cake cannot be hard (4)

I believe the answer is:

easy

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'a piece of cake' is the definition.
(I've seen this before)

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

(Other definitions for easy that I've seen before include "Tranquil" , "Promiscuous" , "Presenting few difficulties" , "Simple or uncomplicated" , "Not rocket science" .)

I've seen this clue in the Evening Standard.

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