Talk indistinctly (6)
'talk indistinctly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mutter that I've seen before include "Talk indistinctly in a low voice - a German mother" , "carp, > grouse and > beef" , "Lack of energy" , "Mumble" , "Talk under one's breath" .)
(Other definitions for mumble that I've seen before include "Speak indistinctly, fail to articulate" , "Speak unclearly" , "indistinct utterance" , "Mutter" , "Speak inarticulately" .)