Obscene material (4)
I believe the answer is:
smut
'obscene material' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for smut that I've seen before include "Salacious material" , "Soot flake or sexual innuendo" , "Small flake of dirt" , "Pornographic material" , "Dirt or immoral talk" .)