Oil reservoir (4)
I believe the answer is:
sump
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">oil reservoir</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 sump that I've seen before include "Water collector" , "It receives the engine oil" , "Pit receiving drainage" , "Engine oil reservoir" , "oil well?" .)