Ploughman's navigational aid (6)
I believe the answer is:
tiller
I believe this is a double definition.
'ploughman's' is the first definition.
(I've seen this before)
'navigational aid' is the second definition.
(bar used for steering on a boat)
(Other definitions for tiller that I've seen before include "One who works the land" , "Bar that manipulates rudder" , "Boat's steering arm" , "Boat's steering bar" , "Handle for turning rudder" .)