In-flight announcement, for short (3)
I believe the answer is:
eta
'in-flight announcement 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 "Militant Basque organisation - Greek letter" , "Letter from Doris" , "when it might reach us?" , "Greek letter - Basque nationalist movement" , "indicating when flight is due" .)