Soot particle (4)
I believe the answer is:
smut
'soot particle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smut that I've seen before include "(Speck) of dirt" , "(Small piece of ) filth" , "Soot flake or sexual innuendo" , "Mild obscenity" , "Sooty matter" .)