Plane's velocity (8)
I believe the answer is:
airspeed
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">plane's velocity</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 airspeed that I've seen before include "that keeps one flying?" , "what Wellington could have got up to" , "Flying velocity" , "Jumbo with such rapidity?" , "Plane's velocity" .)