Indistinct sound (6)
I believe the answer is:
murmur
'indistinct sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for murmur that I've seen before include "Low background noise" , "Mumble" , "Whisper; complain" , "Indistinct mutter" , "Quietly spoken utterance; heart defect" .)