Wood, equally hard (3)
I believe the answer is:
ash
'wood' is the definition.
(wood from an ash tree)
'equally hard' is the wordplay.
'equally' becomes 'as' (synonyms).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'as'+'h'='ASH'
(Other definitions for ash that I've seen before include "Oleaceous tree" , "Common tree" , "Silvery-grey" , "-- Wednesday" , "Fire's remains" .)