Prefix in aviation (4)
I believe the answer is:
aero
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prefix in aviation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for aero that I've seen before include "Prefix denoting air" , "bar" , "Prefix meaning air or aircraft" .)