After a month, publication rejected letter from Athens (5)
I believe the answer is:
gamma
'letter from athens' is the definition.
(letter of the Greek alphabet)
'after a month publication rejected' is the wordplay.
'after' says to put letters next to each other.
'month' becomes 'M' (standard abbreviation).
'publication' becomes 'mag' (abbreviation for magazine).
'rejected' shows that the letters should be reversed in order.
'a'+'m'='am'
'am' put next to 'mag' is 'ammag'.
'ammag' backwards is 'GAMMA'.
(Other definitions for gamma that I've seen before include "Poor mark" , "Third letter of the Greek alphabet" , "Greek character" , "Greek letter G" , "Third class, Gk. letter" .)