Soot upsets little tummies (4)
I believe the answer is:
smut
'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" .)