Greek character is back with good degree (5)
I believe the answer is:
sigma
'greek character' is the definition.
(letter from the Greek alphabet)
'is back with good degree' is the wordplay.
'back' says the letters should be written backwards.
'with' says to put letters next to each other.
'good' becomes 'g' (abbreviation).
'degree' becomes 'ma' (Master of Arts is a type of degree).
'is' in reverse letter order is 'si'.
'si'+'g'+'ma'='SIGMA'
(Other definitions for sigma that I've seen before include "Greek character" , "character abroad" , "Letter from Greece" , "The 18th letter of the Greek alphabet" , "Letter from Athens" .)