Mumble an insult? (4)

I believe the answer is:

slur

I believe this is a double definition.

'mumble' is the first definition.
(thesaurus)

'an insult?' is the second definition.
(thesaurus)

(Other definitions for slur that I've seen before include "Disparaging comment" , "Slight sound" , "Pronounce indistinctly - discredit" , "It's not clear, so to speak" , "Disparage" .)

I've seen this clue in The Telegraph.

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