Soldier’s brought back mother’s letter from Greece (5)
I believe the answer is:
sigma
'letter from greece' is the definition.
(letter from the Greek alphabet)
'soldier's brought back mother's' is the wordplay.
'soldier's' becomes 'gis' (government issue or general issue).
'brought back' says the letters should be written backwards.
'mother' becomes 'ma' (ma is a kind of mother).
'gis' backwards is 'sig'.
'sig'+'ma'='SIGMA'
(Other definitions for sigma that I've seen before include "character in Athens" , "Greek letter indicating summation in maths" , "Gr. letter; rhymes with mark of disgrace" , "How Socrates ended" , "Greek letter signifying sum in maths" .)