Flue residue; obscene material (4)
I believe the answer is:
smut
I believe this clue is a double definition.
'flue residue' is the first definition.
(I've seen this in another clue)
'obscene material' is the second definition.
(I've seen this in another clue)
(Other definitions for smut that I've seen before include "(Speck) of dirt" , "Piece of dirt or salacious talk" , "blue material" , "Pornography or grime" , "Dirt or immoral talk" .)