Low, continuous sound (6)
I believe the answer is:
murmur
'low continuous sound' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for murmur that I've seen before include "Low indistinct sound" , "Soft voices (of discontent)" , "Mumbled expression of malcontent" , "Low continuous sound" , "Quietly spoken utterance; heart defect" .)