One of those easy pieces? (4)

I believe the answer is:

cake

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'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for cake that I've seen before include "Something from Dundee, Eccles or Madeira?" , "Rich item of bakery" , "Compact mass of something" , "Mixture of flour, sugar, eggs and fat" , "Thicken" .)

I've seen this clue in the Evening Standard.

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