Bottom of a boat (4)
I believe the answer is:
keel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bottom of a boat</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)