Sound made by a bird (7)
(Other definitions for whistle that I've seen before include "Make a high-pitched noise by blowing air" , "Pucker up and blow" , "Make nonchalant noise" , "For something, ask in vain" , "Instrument; suit (slang)" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sound made by a bird</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for chirrup that I've seen before include "Bird's noise" , "Chirp" , "Sound made by a bird" , "Tweet" , "Trill" .)