Stomach's turned seeing obscenity (4)
I believe the answer is:
smut
'obscenity' is the definition.
(thesaurus)
'stomach's turned' is the wordplay.
'stomach's' becomes 'tums' ('tum' can be a synonym of 'stomach').
'turned' says the letters should be written in reverse.
'tums' backwards is 'SMUT'.
'seeing' is the link.
(Other definitions for smut that I've seen before include "Cereal disease; filth" , "Salacious material" , "Sooty matter" , "Obscene word(s)" , "Piece of dirt or salacious talk" .)