Obscene stuff turned stomachs (4)
I believe the answer is:
smut
'obscene stuff' is the definition.
(I've seen this before)
'turned stomachs' is the wordplay.
'turned' says the letters should be written in reverse.
'stomachs' becomes 'tums' ('tum' can be a synonym of 'stomach').
'tums' reversed gives 'SMUT'.
(Other definitions for smut that I've seen before include "Lewd material" , "Smudge of soot" , "Sooty matter" , "Small flake of dirt" , "Disease of cereal grass" .)