Crossword Genius

Soot upsets little tummies (4)

Ross

I believe the answer is:

smut

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'soot' is the definition.
(I know that soot can be written as smut)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for smut that I've seen before include "Soot, dirt, grit" , "(Small piece of ) filth" , "Particle of dirt" , "black spot" , "Sooty deposit" .)

I've seen this clue in the Sydney Morning Herald.
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