Wood that may be used for burning when hard (3)
I believe the answer is:
ash
'wood that may be used for burning' is the definition.
'ash' can be an answer for 'wood' (wood from an ash tree). I am not sure about the remainder of the definition.
'when hard' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'hard' becomes 'h' (abbreviation used in pencil classifications).
'as'+'h'='ASH'
Can you help me to learn more?
(Other definitions for ash that I've seen before include "remains after forest fire?" , "Burnt remains; tree" , "Tree; burned residue" , "It comes from volcano" , "deposit" .)