Labs used a chunk (4)
I believe the answer is:
slab
'a chunk' is the definition.
(I've seen this in another clue)
'labs used' is the wordplay.
'used' indicates an anagram.
'labs' with letters rearranged gives 'SLAB'.
(Other definitions for slab that I've seen before include "Thick piece of something" , "Thin flat stone" , "Broad, thick piece of wood, stone" , "Hunk" , "Mortuary table" .)