A necessity in winemaking (4)

I believe the answer is:

must

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'a necessity' is the definition.
(I know that must is a type of essential)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for must that I've seen before include "Smell of dampness or staleness" , "The imperative for mould" , "Fermenting grape juice" , "Unfermented wine" , "Make off with (plane)" .)

I've seen this clue in the Evening Standard.

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