When a plane is due in, for short (3)
I believe the answer is:
eta
'when a plane is due in for short' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eta that I've seen before include "Basque separatist movement" , "Greek letter - Basque nationalist movement" , "when it might reach us?" , "when you can hope to receive it" , "Greek character?" .)