Cigarette residue (3)
I believe the answer is:
ash
'cigarette residue' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ash that I've seen before include "Tree; burnt residue" , "Remains of conflagration" , "What's left after burning" , "Rowan tree is also called the mountain . . ." , "Powdery residue from fire or cigarette" .)