Steering gear made of hard wood (4)
I believe the answer is:
helm
'steering gear' is the definition.
(helm is a kind of steering gear)
'hard wood' is the wordplay.
'hard' becomes 'h'.
'wood' becomes 'elm' (elm is a kind of wood).
'h'+'elm'='HELM'
'made of' is the link.
(Other definitions for helm that I've seen before include "Tiller - part of armour" , "Steering gear" , "Ship's steering apparatus" , "part of boat near stern" , "protection for nut once" .)