Misfit learns about the foreign mudslinger (9)
I believe the answer is:
slanderer
'mudslinger' is the definition.
(thesaurus)
'misfit learns about the foreign' is the wordplay.
'misfit' indicates anagramming the letters (I've seen this in other clues).
'about' indicates putting letters inside.
'the foreign' becomes 'der'.
'learns' anagrammed gives 'slaner'.
'slaner' placed around 'der' is 'SLANDERER'.
(Other definitions for slanderer that I've seen before include "One may be malicious" , "One who defames another" , "One damaging a person's reputation" , "One speaking ill" , "He maligns" .)