Cesspool (4)
I believe the answer is:
sump
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cesspool</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sump that I've seen before include "Engine oil reservoir" , "Pit collecting water/oil" , "Oil store" , "oil well?" , "Crankcase reservoir" .)