Indecent material (4)
I believe the answer is:
smut
'indecent material' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for smut that I've seen before include "Mildly obscene remarks" , "Small soot flake or indecent talk" , "Soot flake; cereal disease" , "Pornographic material" , "Smudge of soot" .)