Nautical spine (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
keel
'nautical 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 "Lengthwise structure of a ship" , "Structural member of a ship" , "Bottom of boat" , "Cool, or it used to be" , "Low part of ship" .)