Spoke indistinctly in a low voice (8)
I believe the answer is:
muttered
'spoke indistinctly in a low voice' is the definition.
(I've seen this before)
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" , "Talked in a barely audible voice" , "Talked indistinctly in a low voice" , "Murmured, mumbled" .)