Crankcase reservoir (4)
I believe the answer is:
sump
'crankcase reservoir' is the definition.
(I've seen this before)
This is all the clue.
(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" .)