It causes fermentation, as in a 'still' (5)

I believe the answer is:

yeast

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'it causes fermentation' is the definition.
The definition suggests a singular noun which matches the answer.

'as in a still' is the wordplay.
'in' indicates putting letters inside.
'a still' becomes 'yet' (I've seen this before).
'as' going inside 'yet' is 'YEAST'.

Can you help me to learn more?

(Other definitions for yeast that I've seen before include "Brewer" , "fermenter" , "Substance used to raise dough etc" , "lifting agent" , "Fungi used in bakery" .)

I've seen this clue in the Evening Standard.

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