In-flight announcement, briefly (3)
I believe the answer is:
eta
'in-flight announcement briefly' 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 "When you'd expect to see" , "European separatists, initially" , "Arrival roughly indicated" , "giving expected termination of journey" , "Something like 10 [RHO]" .)