An absolute necessity in winemaking (4)

I believe the answer is:

must

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'an absolute' is the definition.
(thesaurus)

I can't explain the remainder of the clue.

(Other definitions for must that I've seen before include "Prerequisite" , "Mould - grape juice during fermentation" , "The imperative for mould" , "Fermenting grape juice" , "Grape juice ready for fermentation" .)

I've seen this clue in the Evening Standard.

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