Greek character’s estimated time of arrival (3)
I believe the answer is:
eta
This clue is a double definition.
'greek character's' is the first definition.
(letter from the Greek alphabet)
'estimated time of arrival' is the second definition.
(abbreviation)
(Other definitions for eta that I've seen before include "Militant Basque organisation - Greek letter" , "when it might reach us?" , "Greek H" , "when you can hope to receive it" , "Basque separatist movement" .)