Subdued complaint (6)
I believe the answer is:
murmur
'subdued complaint' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for murmur that I've seen before include "Whisper; complain" , "Low, indistinct speech" , "Quiet utterance" , "Speak in a low voice, whisper" , "Low background noise" .)