Obscenity turns stomachs (4)
I believe the answer is:
smut
'obscenity' is the definition.
(I know that obscenity can be written as smut)
'turns stomachs' is the wordplay.
'turns' says the letters should be written backwards.
'stomachs' becomes 'tums' ('tum' can be a synonym of 'stomach').
'tums' in reverse letter order is 'SMUT'.
(Other definitions for smut that I've seen before include "Soot flake; cereal disease" , "Indecent talk or piece of soot" , "Salacious material" , "Lewd material" , "Soot, dirt, grit" .)