Obscene material turning stomachs (4)
I believe the answer is:
smut
'obscene material' is the definition.
(I've seen this before)
'turning stomachs' is the wordplay.
'turning' shows that the letters should be reversed in order.
'stomachs' becomes 'tums' ('tum' can be a synonym of 'stomach').
'tums' backwards is 'SMUT'.
(Other definitions for smut that I've seen before include "black spot" , "Obscene word(s)" , "Dirty object or talk" , "Mild obscenity" , "Cereal disease; filth" .)