Letter returned by American soldiers (5)
I believe the answer is:
sigma
'letter' is the definition.
(letter of the Greek alphabet)
'returned by american soldiers' is the wordplay.
'returned' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'american' becomes 'Am' (abbreviation in some contexts).
'soldiers' becomes 'GIs' (GI is an informal term for a US soldier).
'am' put next to 'gis' is 'amgis'.
'amgis' reversed gives 'SIGMA'.
(Other definitions for sigma that I've seen before include "letter to Greeks" , "Foreign character" , "Allowing no relaxation" , "Greek letter signifying sum in maths" , "Greek letter used as summation sign" .)