Hunk of concrete (4)
I believe the answer is:
slab
'hunk of concrete' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slab that I've seen before include "Large block of stone or concrete" , "Flat mass of stone" , "Piece, e.g. of cake" , "24 tinnies [STRINE] < in a wooden hut [STRINE]" , "Thick piece of something" .)