When a plane is due, for short (3)
I believe the answer is:
eta
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">when a plane is due for short</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 eta that I've seen before include "Arrival roughly indicated" , "Greek character?" , "Greek letter - time due (abbr)" , "Greek H" , "When one is due" .)