Chimney dust (4)
I believe the answer is:
soot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">chimney dust</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)