Organ of flight (4)
I believe the answer is:
wing
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">organ of flight</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 wing that I've seen before include "Flank; move quickly" , "Chicken portion" , "Aircraft lift provider" , "Flying organ" , "Limb - faction" .)