Sing quaveringly (6)
I believe the answer is:
warble
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sing quaveringly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for warble that I've seen before include "Sing with trilling sound, like bird" , "Sing like a thrush" , "Sing - type of fly" , "Sing with trilling note - like bawler" , "Trill, yodel" .)