Chimney residue (4)
I believe the answer is:
soot
'chimney residue' 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 "Carbon dust from fire" , "Residue from burning" , "Chimney coating" , "Fine carbon deposit" , "Something stuck down the chimney" .)