Burnt residue (3)
I believe the answer is:
ash
'burnt residue' is the definition.
(residue of a fire)
This is the entire clue.
(Other definitions for ash that I've seen before include "Tree - effect of burning" , "Knotty - - (Ken Dodd)" , "Tree with winged seeds" , "Oleaceous tree" , "Get the residue" .)