Ship's backbone (4)
I believe the answer is:
keel
'ship's backbone' 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 "Long part on a boat's bottom" , "Support on which ship is built" , "Longitudinal structure supporting the frame of a vessel" , "Long timber" , "Structural member of a ship" .)