Possible Clues:

• One slip ruined Greek character
• Fifth star in a constellation
• Poles in error get a letter for the Greeks
• Greek letter records strange lion
• Greek character's no problem speaking English? Just the reverse
• Fifth of 24 letters
• Leader of ancient Ephesus?
• Greek character taking drug with the ability to say 'sausages' backwards?
• A letter in retinol is perfectly reversed
• Delta-zeta connection