Like hard wood (3)
I believe the answer is:
ash
'wood' is the definition.
(wood from an ash tree)
'like hard' is the wordplay.
'like' becomes 'as'.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'as'+'h'='ASH'
(Other definitions for ash that I've seen before include "Powdery residue from fire or cigarette" , "Tree with pale wood" , "Fraxinus" , "Hardwood" , "Knotty - - (Ken Dodd)" .)