Volcanic dust (3)
I believe the answer is:
ash
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">volcanic dust</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ash that I've seen before include "Fraxinus" , "Tree; fire residue" , "Remains of conflagration" , "Knotty - - (Ken Dodd)" , "Powdery residue from fire or cigarette" .)