Thick piece of stone (4)
I believe the answer is:
slab
'thick piece of stone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for slab that I've seen before include "Piece, e.g. of cake" , "Large block of stone or concrete" , "Chunk of wood" , "Thin flat stone" , "Flat piece of stone or 20" .)