Stomachs turned by dirty joke (4)
I believe the answer is:
smut
'dirty joke' is the definition.
'smut' can be an answer for 'dirty' (thesaurus). I'm not certain of the 'joke' bit.
'stomachs turned' is the wordplay.
'stomachs' becomes 'tums' ('tum' can be a synonym of 'stomach').
'turned' is a reversal indicator.
'tums' written backwards gives 'SMUT'.
'by' is the link.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "Small soot flake or indecent talk" , "Indecent talk or piece of soot" , "Dirty stuff" , "Mildly obscene remarks" , "blue material" .)