Burnt matter (3)
I believe the answer is:
ash
'burnt matter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ash that I've seen before include "Tree; cinders" , ". . . . [FOREST] One of which" , "Remnant from fire" , "Silvery-grey" , "In fireplace it may be" .)