Arson evidence (3)
I believe the answer is:
ash
'arson evidence' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ash that I've seen before include "In fireplace it may be" , "Powdery residue from fire or cigarette" , "Grey colour" , ". . . . [FOREST] One of which" , "Tree - effect of burning" .)