Possible Clues:

• Sigma successor?
• Hellenic T
• Alpha ___ Omega (fraternity founded in 1865)
• Cross and tense when failing to finish
• St. Anthony's cross, e.g.
• With 45-Down, Advent symbol
• 19th letter of Greek alphabet
• T as in ''Telemachus''
• 19th letter of the Greek alphabet
• Tense before finishing letter from foreigner