Envelope part (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">envelope part</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for flap that I've seen before include "Tab - tizzy - move wings" , "Minor panic; tab" , "Fluster" , "Adjustable part of wing - panic" , "Moving surface on wing" .)