Bottom of the ocean (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bottom of the ocean</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for seabed that I've seen before include "Davy Jones's locker" , "marine base" , "The ocean floor, not for sleeping" , "Bottom of the ocean" , "Debase (anag)" .)