Mostly vulgar filth (4)
I believe the answer is:
crud
'filth' is the definition.
(thesaurus)
'mostly vulgar' is the wordplay.
'mostly' means to remove the last letter.
'vulgar' becomes 'crude' ('crude' can be a synonym of 'vulgar').
'crude' with its last letter taken away is 'CRUD'.
(Other definitions for crud that I've seen before include "Disgustingly foul substance" , "worthless stuff" , "That's disgusting" , "Disgusting waste matter" , "radioactive waste" .)