Crossword Genius

Indistinctly make an insult (4)

Ross

I believe the answer is:

slur

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

I cannot understand how the remainder of the clue works.

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(Other definitions for slur that I've seen before include "Slight - smear" , "Speak as drunk perhaps" , "Slight sound" , "Disparage" , "Slight - mumble" .)

I've seen this clue in The Daily Express.
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