Possible Clues:

• The 18th letter of the Greek alphabet
• Greek letter signifying sum in maths
• Letter from Athens dismissing tour leader in disgrace
• Greek letter corresponding to S
• Allowing no relaxation
• 'The Sweetheart of -- Chi'
• How Socrates ended - in disgrace, no end of it
• Father and son writing up about first of Grecians — or his character?
• Greek character drops temperature in part of plant
• Character going to extremes in Sophocles