Letter from Greece going abroad, mailed miles afield initially (5)
I believe the answer is:
gamma
'letter from greece' is the definition.
(letter from the Greek alphabet)
'going abroad mailed miles afield initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'going abroad mailed miles afield' is 'GAMMA'.
(Other definitions for gamma that I've seen before include "Mediocre grade" , "Letter from Greece" , "Third letter of the Greek alphabet" , "Greek letter G" , "Greek character" .)