Son, 2 having cut top off a piece of cake (6)

I believe the answer is:

simple

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

I don't understand the remainder of the clue.

(Other definitions for simple that I've seen before include "Elementary" , "Straightforward, not elaborate" , "Easy; herb" , "Easily done/understood" , "Penniless pie seeker" .)

I've seen this clue in The Guardian.

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