Stomach's turned by lewd material (4)
I believe the answer is:
smut
'lewd material' is the definition.
(I've seen this in another clue)
'stomach's turned' is the wordplay.
'stomach's' becomes 'tum's' ('tum' can be a synonym of 'stomach').
'turned' is a reversal indicator.
'tums' in reverse letter order is 'SMUT'.
'by' is the link.
(Other definitions for smut that I've seen before include "Salacious material" , "blue material" , "(Speck) of dirt" , "Dirt or immoral talk" , "Dirty object or talk" .)