Campfire residue (3)
I believe the answer is:
ash
'campfire residue' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ash that I've seen before include "Knotty - - (Ken Dodd)" , "It no longer burns" , "remains after forest fire?" , "Burnt material; tree" , "Wooden" .)