Inflight announcement (3)
I believe the answer is:
eta
'inflight announcement' 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 "indicating when flight is due" , "When you'd expect to see" , "Greek letter - time due (abbr)" , "Militant Basque organisation - Greek letter" , "Greek letter E; estimated time of arrival (abbr.)" .)