Kind of mash to ban outright? (4)

I believe the answer is:

bran

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'kind of mash' is the definition.
'bran' can be an answer for 'mash' (thesaurus). I am not certain of the 'kind of' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for bran that I've seen before include "Dietary fibre source" , "Husks valued as fibre" , "Coarse meal for horse food" , "Milled grain husk" , "Separated grain husks" .)

I've seen this clue in the Evening Standard.

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