That’s it, folks! (3)

I believe the answer is:

all

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'that's' is the definition.
Both the answer and definition are adverbs.
Perhaps there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for all that I've seen before include "Passage in a formal garden" , "Entirely; everyone" , "Total" , "Avenue, walk or garden path" , "and 16 Across and 23 Across: Even so" .)

I've seen this clue in the Evening Standard.

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