Boat's backbone (4)
I believe the answer is:
keel
'boat's 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 "Flat-bottomed, coal carrying vessel" , "Long base of a ship" , "Lengthwise structure of a ship" , "Earth (Latin)" , "Fore-and-aft base to a ship's hull" .)