Smut (5)
I believe the answer is:
filth
'smut' is the definition.
(I know that smut can be written as filth)
This is all the clue.
(Other definitions for filth that I've seen before include "Muck" , "Dirt, obscene talk" , "Disgusting dirt or obscenity" , "Squalor" , "Real dirt or obscenity" .)