Ship's spine (4)
I believe the answer is:
'ship's spine' is the definition.
(I've seen this in another clue)
This is the entire 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" .)