Slanderous comment (5)
(Other definitions for smear that I've seen before include "Smother" , "Calumniate" , "Besmirch" , "Blot or daub" , "Spread a greasy or sticky substance over something" .)
'slanderous comment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slur that I've seen before include "Musical sign: "Run together"" , "Slight - mumble" , "Discredit, say reputation" , "Contentious allegation" , "Slight - mark on music" .)