The task is halved in a letter from Greece (5)
I believe the answer is:
theta
'a letter from greece' is the definition.
(letter from the Greek alphabet)
'the task is halved' is the wordplay.
'is halved' suggests halving the letters.
'task' cut in half is 'ta'.
'the'+'ta'='THETA'
'in' acts as a link.
(Other definitions for theta that I've seen before include "Foreign letter" , "Eighth letter of Greek alphabet" , "letter from Argos?" , "letter from abroad" , "Greek letter used in trigonometry" .)