Reservoir to sketch — later you must leave (4)

I believe the answer is:

sump

'reservoir' is the definition.
(I know that sump is a type of reservoir)

I don't understand the remainder of the clue.

(Other definitions for sump that I've seen before include "Hole collecting water for pump" , "Lubrication reservoir" , "Pit collecting water/oil" , "Crankcase reservoir" , "lowest point of mine" .)

I've seen this clue in The Times.

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