Trill (6)
I believe the answer is:
warble
'trill' is the definition.
(I know that trill can be written as warble)
This is all the clue.
(Other definitions for warble that I've seen before include "Sing with trilling sound, like bird" , "Sing with trilling note like a bird" , "Sing with trilling note - like bawler" , "Trill, yodel" , "Trill or sing like bird" .)