French friends turned back, receiving good letter from Greece (5)
I believe the answer is:
sigma
'letter from greece' is the definition.
(letter from the Greek alphabet)
'french friends turned back receiving good' is the wordplay.
'french friends' becomes 'amis' (I've seen this in another clue).
'turned back' says the letters should be written in reverse.
'receiving' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'amis' reversed gives 'sima'.
'sima' enclosing 'g' is 'SIGMA'.
(Other definitions for sigma that I've seen before include "Eighteenth letter of the Greek alphabet" , "How Socrates ended" , "Athenian character" , "Letter from Greece" , "Gr. letter; rhymes with mark of disgrace" .)