The task is halved with 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' means to take half the letters.
'task' cut in half is 'ta'.
'the'+'ta'='THETA'
'with' acts as a link.
(Other definitions for theta that I've seen before include "Greek character" , "Greek letter used in trigonometry" , "Letter from Greece" , "Greek letter equivalent to th" , "letter to Greeks" .)