The wheel is of hard wood! (4)
I believe the answer is:
helm
'the wheel' is the definition.
(thesaurus)
'hard wood' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'wood' becomes 'elm' (elm is a kind of wood).
'h'+'elm'='HELM'
'is of' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for helm that I've seen before include "Steering gear" , "Tiller; old headgear" , "Tiller, steering wheel" , "Steering mechanism" , "Tiller; steer" .)