Somewhat obscene material turning stomachs over (4)
I believe the answer is:
smut
'somewhat obscene material turning' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'stomachs over' is the wordplay.
'stomachs' becomes 'tums' ('tum' can be a synonym of 'stomach').
'over' is a reversal indicator.
'tums' in reverse letter order is 'SMUT'.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "black spot" , "Erotica" , "Dirt or immoral talk" , "Mildly obscene remarks" , "Dirty object or talk" .)