Under root of tree, spare wood (5)
I believe the answer is:
ebony
'wood' is the definition.
(ebony is a kind of wood)
'under root of tree spare' is the wordplay.
'under' means one lot of letters go next to another.
'root of' suggests the final letters (in a down clue, the letter at the bottom).
'spare' becomes 'bony' (both can mean very thin).
The last letter of 'tree' is 'e'.
'e' put next to 'bony' is 'EBONY'.
(Other definitions for ebony that I've seen before include "Hard dark-coloured wood" , "Hard wood used for black piano keys and in cabinetwork" , "Wood - Boyne (anag)" , "'Hard, dark wood (5)'" , "Black hardwood" .)