Fool spreading filth was cut short (7)
I believe the answer is:
halfwit
'fool' is the definition.
(thesaurus)
'spreading filth was cut short' is the wordplay.
'spreading' is an anagram indicator.
'cut short' means to remove the last letter.
'filth'+'was'='filthwas'
'filthwas' with its last letter taken away is 'filthwa'.
'filthwa' anagrammed gives 'HALFWIT'.
(Other definitions for halfwit that I've seen before include "Charlie?" , "Idiot" , "Mug" , "Being stupid" , "One only 50% all there?" .)