Small dog’s large slice of cake (4)
I believe the answer is:
slab
'large slice of cake' is the definition.
(I've seen this before)
'small dog's' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'dog' becomes 'lab' (short for labrador**).
's'+'lab'='SLAB'
(Other definitions for slab that I've seen before include "Thick flat piece of stone" , "Flat broad piece as of stone" , "Flat piece of stone or 20" , "Mortuary table" , "Flat mass of stone" .)