Trill (6)
I believe the answer is:
warble
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">trill</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that trill can be written as warble)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)