Chimney dust (4)
I believe the answer is:
soot
'chimney dust' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for soot that I've seen before include "Deposited carbon" , "Stuff from chimneys" , "Carbon deposit from burning" , "Something stuck down the chimney" , "Chimney coating" .)