Sap in sewer (5)
I believe the answer is:
This clue is a double definition.
'sap' is the first definition.
'sewer' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for drain that I've seen before include "Exhaust - pipe" , "Exhaust (of liquid)" , "Discharge pipe" , "Filter" , "Waste pipe" .)