Obscene material corporations rejected (4)
I believe the answer is:
smut
'obscene material' is the definition.
(I've seen this in another clue)
'corporations rejected' is the wordplay.
'corporations' becomes 'tums' (corporation can humorously mean stomach fat).
'rejected' says the letters should be written backwards.
'tums' in reverse letter order is 'SMUT'.
(Other definitions for smut that I've seen before include "Smudge of soot" , "Sooty matter" , "Particle of dirt" , "black spot" , "Bit of soot or dirty talk" .)