Insult indistinctly (4)

I believe the answer is:

slur

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'insult' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for slur that I've seen before include "Speak as drunk perhaps" , "Pronounce indistinctly or cast abuse on reputation" , "Slight; blur" , "Slight lack of distinction in speech" , "Speak indistinctly, as a drunk might" .)

I've seen this clue in The Daily Express and The Mirror.

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