Muttering (6)
I believe the answer is:
murmur
'muttering' is the definition.
(I know that muttering can be written as murmur)
This is the entire clue.
(Other definitions for murmur that I've seen before include "Speak in a low voice, whisper" , "Little noise" , "Low background noise" , "Quietly spoken utterance; heart defect" , "Abnormal sound made by heart" .)