Volcanic dust (3)
I believe the answer is:
ash
'volcanic dust' is the definition.
(I've seen this before)
This is all the clue.
(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" .)