It may well be a piece of cake (4)

I believe the answer is:

slab

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'piece of cake' is the definition.
(large slice of cake)

I cannot understand how the rest of the clue works.

(Other definitions for slab that I've seen before include "Piece of paving" , "Broad, thick piece of wood, stone" , "Hunk" , "24 tinnies [STRINE] < in a wooden hut [STRINE]" , "Thick flat piece of stone" .)

I've seen this clue in the Evening Standard.

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