It's a piece of cake! (4)
I believe the answer is:
slab
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">it's a piece of cake</span>' is the definition.<br/ ><span class="explanation-format__explanation">(large slice of cake)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)