When a plane is due, for short (3)
I believe the answer is:
eta
'when a plane is due for short' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)