Speak impolitely about Norfolk town (4)

I believe the answer is:

diss

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

I don't understand the rest of the clue.

(Other definitions for diss that I've seen before include "Algerian grass" , "Be impolite" , "Eastern town" , "Norfolk town" , "East Anglian town" .)

I've seen this clue in The Independent.

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