Fungus on grapes (5)

I believe the answer is:

yeast

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'fungus' is the definition.
(I know that yeast is a type of fungus)

I don't understand the remainder of the clue.

(Other definitions for yeast that I've seen before include "Rising agent" , "What makes bread rise" , "Fungi used in bakery" , "It's put in to raise" , "As yet (anag)" .)

I've seen this clue in the Evening Standard.

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