Corporations rejected rude material (4)
I believe the answer is:
smut
'rude material' is the definition.
(smut can mean pornographic material)
'corporations rejected' is the wordplay.
'corporations' becomes 'tums' (corporation can humorously mean stomach fat).
'rejected' says the letters should be written in reverse.
'tums' written backwards gives 'SMUT'.
(Other definitions for smut that I've seen before include "Indecent talk or piece of soot" , "(Small piece of ) filth" , "Soot flake; fungal disease" , "Sooty matter" , "Small bit of soot" .)