Backbone of a ship (4)
I believe the answer is:
keel
'backbone of a ship' 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" , "Earth (Latin)" , "Ship's bottom" , "Fore-and-aft base to a ship's hull" , "Coal-carrying vessel" .)