Possible Clues:

• ___ Society (super-high I.Q. group)
• A thousand soldiers backed local opening in Salonika
• Gr. letter; rhymes with mark of disgrace
• Greek letter indicating summation in maths
• American soldier's sent back letter
• One of those letters from abroad soldiers sent back to mum
• Letter from Greece is sent up with note to mother
• The American serviceman's got back to his mother with a letter from Greece
• Calculus symbol
• Whichever way you look at it, the end of Socrates