Chimney duct (4)
I believe the answer is:
'chimney duct' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flue that I've seen before include "Chimney channel" , "Vent pipe for smoke, etc" , "Passage for air" , "Chimney duct" , "Chimney-duct" .)