Low, continuous sound (6)
I believe the answer is:
murmur
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">low continuous sound</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)