Remains after burning tree (3)
I believe the answer is:
ash
This clue is a double definition.
'remains after burning' is the first definition.
(remains of a fire)
'tree' is the second definition.
(ash is a kind of tree)
(Other definitions for ash that I've seen before include "What's often left after something 1 Ac" , "Knotty - - (Ken Dodd)" , "Rowan tree is also called the mountain . . ." , "Remnant from fire" , "What's left of the fire" .)