Speck of soot or dirt (4)

I believe the answer is:

smut

I believe this clue is a double definition.

'speck of soot' is the first definition.
(I've seen this before)

'dirt' is the second definition.
(I've seen this before)

(Other definitions for smut that I've seen before include "Dirt or obscenity" , "Sooty deposit" , "Small soot flake or indecent talk" , "Obscene stuff" , "Soot; obscenities" .)

I've seen this clue in the Evening Standard.

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