London's main airport (8)
I believe the answer is:
heathrow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">london's main airport</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 heathrow that I've seen before include "our counterpart of Charles de Gaulle" , "site for airport" , "London's busiest airport" , "London's main airport" , "lots of movement through the air here" .)