It's a piece of cake! (4)
I believe the answer is:
slab
'it's a piece of cake' is the definition.
(large slice of cake)
This is all the clue.
(Other definitions for slab that I've seen before include "Hunk" , "Thick flat piece of stone" , "Piece, e.g. of cake" , "Thin flat stone" , "Large block of stone or concrete" .)