Possible Clues:

• Agism (anag.)
• Married soldier's sent back a letter from abroad
• Rho-tau link
• Mathematical symbol: short symbol written by scholar
• From returning US soldiers, mother getting foreign letter
• English writer upset about first letter from Greece — or a later one
• Mathematician’s summation symbol
• Greek character American soldiers backed
• Symbol of stock volatility
• American soldiers in retreat showing character