Smoke outlet (4)
(Other definitions for flue that I've seen before include "smoky passage" , "Chimney duct" , "Smoke duct" , "Conduit for smoke" , "Outlet for smoke or gas" .)
'smoke outlet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chimney that I've seen before include "A cleft for climbing" , "Smoke outlet" , "Smoke pipe; rock cleft" , "Smoke vent" , "Flue" .)