Sing quaveringly (6)
I believe the answer is:
warble
'sing quaveringly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)