The number spoken was mumbled (8)

I believe the answer is:

muttered

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'mumbled' is the definition.
('mutter' can be a synonym of 'mumble')

I cannot understand how the rest of the clue works.

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(Other definitions for muttered that I've seen before include "Spoke indistinctly in a low voice" , "Talked indistinctly in a low voice" , "Spoke in low unclear tone" , "Murmured, mumbled" , "Spoke in low unclear voice, uttered subdued grumble" .)

I've seen this clue in the Evening Standard.

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