When business is doing so, things are obviously not quiet (7)

I believe the answer is:

booming

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'quiet' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?

I don't understand how the remainder of the clue works.

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(Other definitions for booming that I've seen before include "as 26 [BITTERN] may be" , "Doing well financially" .)

I've seen this clue in the Evening Standard.

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