Bit of dirt, stomach’s upset (4)
I believe the answer is:
smut
'bit of dirt' is the definition.
'smut' can be an answer for 'dirt' (I've seen this in another clue). I'm not certain of the 'bit of' bit.
'stomach's upset' is the wordplay.
'stomach's' becomes 'tum's' ('tum' can be a synonym of 'stomach').
'upset' is a reversal indicator.
'tums' back-to-front is 'SMUT'.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "Pornographic material" , "Piece of dirt or salacious talk" , "Obscene word(s)" , "Mildly obscene remarks" , "Sooty deposit" .)