American soldiers in retreat showing character (5)
I believe the answer is:
sigma
'character' is the definition.
(sigma is a kind of character)
'american soldiers in retreat' is the wordplay.
'american' becomes 'Am' (abbreviation in some contexts).
'soldiers' becomes 'GIs' (GI is an informal term for a US soldier).
'in retreat' is a reversal indicator.
'am'+'gis'='amgis'
'amgis' reversed gives 'SIGMA'.
'showing' is the link.
(Other definitions for sigma that I've seen before include "Letter from Athens" , "Greek character" , "Greek letter used as summation sign" , "character in Corfu" , "character in Athens" .)