Hard wood needed for rudder (4)
I believe the answer is:
helm
'rudder' is the definition.
(I've seen this before)
'hard wood needed' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'wood' becomes 'elm' (elm is a type of tree).
'needed' is a charade indicator (letters next to each other).
'h'+'elm'='HELM'
'for' is the link.
(Other definitions for helm that I've seen before include "Position of steerer" , "Tiller; steer" , "Part of ship" , "Steering gear" , "part of boat near stern" .)