Many spoke but mumbled (8)
I believe the answer is:
muttered
'mumbled' is the definition.
('mutter' can be a synonym of 'mumble')
'many spoke' is the wordplay.
'many' becomes 'm'.
'spoke' becomes 'uttered' ('utter' can be a synonym of 'speak').
'm'+'uttered'='MUTTERED'
'but' acts as a link.
(Other definitions for muttered that I've seen before include "Spoke in low unclear voice, uttered subdued grumble" , "Talked in a barely audible voice" , "Talked indistinctly in a low voice" , "Murmured, mumbled" , "Spoke in low unclear tone" .)