Sooty shaft (4)
I believe the answer is:
flue
'sooty shaft' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flue that I've seen before include "Conduit for smoke" , "Duct for smoke/gases" , "Air vent" , "Chimney shaft" , "Fume vent" .)