Steering apparatus (4)
'steering apparatus' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rudder that I've seen before include "Something used for steering" , "It steers a ship" , "Boat's steering mechanism" , "Control surface on plane's tail" , "Boat steerer" .)
'steering apparatus' is the definition.
(as in the helm on a boat)
This is all the clue.
(Other definitions for helm that I've seen before include "Part of ship" , "Tiller, steering wheel" , "Tiller - part of armour" , "Ship's steering apparatus" , "Wheel that controls ship's rudder" .)