Speck of soot (4)
I believe the answer is:
smut
'speck of soot' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for smut that I've seen before include "Cereal disease; filth" , "Dirt or immoral talk" , "Dirty stuff" , "Dirty jokes - black mark" , "Dirt or pornography" .)