Sooty shaft (4)
'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" .)
(Other definitions for chimney that I've seen before include "Smoke vent" , "Father Christmas's entry?" , "Smoke pipe; rock cleft" , "Smoker on the roof?" , "Flue" .)