Ship's spine (4)
I believe the answer is:
keel
'ship's spine' 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" , "Part of ship's hull" , "Support on which ship is built" , "Longitudinal structure supporting the frame of a vessel" , "Fore-and-aft base to a ship's hull" .)