Steering apparatus made of hard wood (4)
I believe the answer is:
helm
'steering apparatus' is the definition.
(as in the helm on a boat)
'hard wood' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'wood' becomes 'elm' (elm is a type of tree).
'h'+'elm'='HELM'
'made of' acts as a link.
(Other definitions for helm that I've seen before include "Position of leadership" , "Tiller, steering wheel" , "Steersman's place" , "Tiller; old headgear" , "Ship's steering apparatus" .)