Greek character American soldiers backed (5)
I believe the answer is:
sigma
'greek character' is the definition.
(letter from the Greek alphabet)
'american soldiers backed' is the wordplay.
'american' becomes 'Am' (abbreviation in some contexts).
'soldiers' becomes 'GIs' (GI is an informal term for a US soldier).
'backed' says the letters should be written in reverse.
'am'+'gis'='amgis'
'amgis' in reverse letter order is 'SIGMA'.
(Other definitions for sigma that I've seen before include "Greek letter corresponding to S" , "Athenian character" , "Letter from Athens" , "How Socrates ended" , "local opening in Salonika" .)