Boat backbone (4)
I believe the answer is:
keel
'boat backbone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for keel that I've seen before include "Beam on the hull of a vessel" , "Vessel stabiliser" , "Low part of ship" , "Structural member of a ship" , "Part of ship's frame" .)