Epithet always contains a Greek letter (5)
I believe the answer is:
theta
'a greek letter' is the definition.
(letter from the Greek alphabet)
'epithet always contains' is the wordplay.
'contains' indicates the answer is hidden within the clue.
'THETA' can be found hidden inside 'epithet always'.
(Other definitions for theta that I've seen before include "letter from abroad" , "Make amends for" , "letter to Greeks" , "Greek character" , "Letter from Greece" .)