Mutter (6)
I believe the answer is:
mumble
'mutter' is the definition.
(I know that mutter can be written as mumble)
This is all the clue.
(Other definitions for mumble that I've seen before include "indistinct utterance" , "Speak indistinctly, fail to articulate" , "Speak unclearly" , "Speak inarticulately" , "Mutter" .)