Basins
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">basins</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know this)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sinks that I've seen before include "Declines markedly" , "Basins" , "Goes down" , "Dips" , "Scuttles" .)