All that's left of tree?
I believe the answer is:
ash
'tree?' is the definition.
(ash is a kind of tree)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for ash that I've seen before include "It comes from volcano" , "at village near Whitchurch in Shropshire" , "Knotty - - (Ken Dodd)" , "Residue after burning" , "Hardwood" .)