Bottom of a boat (4)
I believe the answer is:
keel
'bottom of a boat' 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 "Support on which ship is built" , "Longitudinal structure supporting the frame of a vessel" , "ridge" , "Part of ship's frame" , "Beam on the hull of a vessel" .)