Possible Clues:

• Air density symbol
• Greek letter that looks like a "P"
• P, for Plato
• Greek alphabet's 17th letter
• Pythagoras's P
• Greek 'P'
• Part of the Greek alphabet
• Greek letter that sounds like the end of 16-, 22-, 36-, 47- or 58-Across
• Pi's successor
• Letter from the Corinthians