Married soldier's sent back a letter from abroad (5)
I believe the answer is:
sigma
'letter from abroad' is the definition.
(letter of the Greek alphabet)
'married soldier's sent back a' is the wordplay.
'married' becomes 'm'.
'soldier's' becomes 'GI's' (GI can mean a US soldier).
'sent back' says the letters should be written in reverse.
'm'+'gis'='mgis'
'mgis' back-to-front is 'sigm'.
'sigm'+'a'='SIGMA'
(Other definitions for sigma that I've seen before include "character in Athens" , "Allowing no relaxation" , "Letter from Athens" , "Foreign character" , "Greek letter signifying sum in maths" .)