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