Mumbled (8)
I believe the answer is:
muttered
'mumbled' is the definition.
('mutter' can be a synonym of 'mumble')
This is all the clue.
(Other definitions for muttered that I've seen before include "Spoke indistinctly in a low voice" , "Spoke in low unclear voice, uttered subdued grumble" , "Spoke in low unclear tone" , "Talked in a barely audible voice" , "Talked indistinctly in a low voice" .)