Rudder made of hard wood (4)
I believe the answer is:
helm
'rudder' is the definition.
(thesaurus)
'hard wood' is the wordplay.
'hard' becomes 'h'.
'wood' becomes 'elm' (elm is a kind of wood**).
'h'+'elm'='HELM'
'made of' acts as a link.
(Other definitions for helm that I've seen before include "Tiller; steer" , "Steer (a boat/ship)" , "protection for nut once" , "Steering gear" , "Wheel that controls ship's rudder" .)