Crankcase reservoir (4)
I believe the answer is:
sump
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">crankcase 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 "Hole collecting water for pump" , "Crankcase reservoir" , "Lubrication reservoir" , "Pit receiving drainage" , "It receives the engine oil" .)