Exhaust pipe (5)
I believe this is a double definition.
'exhaust' is the first definition.
(I know that drain is a more specific form of exhaust)
'pipe' is the second definition.
(I know that drain is a type of pipe)
(Other definitions for drain that I've seen before include "Something using up resources" , "Discharge pipe" , "Waste channel/pipe" , "Deprive gradually of strength or resources" , "Deprive of vitality" .)
(Other definitions for flue that I've seen before include "Conduit for smoke" , "Tube" , "Small chimney" , "Fuel (anag.)" , "Chimney channel" .)