Waste pipe (5)
'waste pipe' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for drain that I've seen before include "Waste channel" , "Waste channel/pipe" , "Bleed" , "Drink dry" , "Something using up resources" .)
(Other definitions for sewer that I've seen before include "Plier of needle; drain" , "It drains" , "Channel to carry away waste water and matter" , "Waste-pipe" , "Pipe for waste matter from houses, factories etc" .)