Letter from abroad, estimated time of arrival on Thursday (5)
I believe the answer is:
theta
'letter from abroad' is the definition.
(letter of the Greek alphabet)
'estimated time of arrival on thursday' is the wordplay.
'estimated time of arrival' becomes 'eta' (abbreviation).
'on' is a charade indicator (letters next to each other).
'thursday' becomes 'Th' (common abbreviation).
'eta' after 'th' is 'THETA'.
(Other definitions for theta that I've seen before include "Homeric character" , "Letter before iota" , "character in Euripides" , "Greek letter equivalent to th" , "letter from abroad" .)