Letter from Greece among post he takes (5)
I believe the answer is:
theta
'letter from greece' is the definition.
(letter from the Greek alphabet)
'among post he takes' is the wordplay.
'among' indicates the answer is hidden within the clue.
'THETA' is hidden within 'post he takes'.
(Other definitions for theta that I've seen before include "Make amends for" , "8th letter of the Greek alphabet" , "Greek letter used in trigonometry" , "character in Marathon" , "Character from overseas" .)