Quiet sound as a double knocked back? (6)
I believe the answer is:
murmur
'quiet sound' is the definition.
'murmur' can be an answer for 'sound' (murmur is a kind of sound). I'm not certain of the 'quiet' bit.
'a double knocked back?' is the wordplay.
'a double' indicates the doubling of some letters.
'knocked back?' becomes 'mur' (I can't explain this - if you can you should give a lot more credence to this answer).
'mur' doubled is 'MURMUR'.
'as' is the link.
Can you help me to learn more?
(Other definitions for murmur that I've seen before include "Soft voices (of discontent)" , "Speak in a low voice, whisper" , "Speak quietly" , "Mumbled expression of malcontent" , "Subdued complaint" .)