Concrete chunk (4)
I believe the answer is:
slab
'concrete chunk' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slab that I've seen before include "Large thick slice (of cake etc)" , "Flat broad piece as of stone" , "Large piece (of chocolate, stone, etc)" , "Large block of stone or concrete" , "Paving stone" .)