Possible Clues:

• Greek character bungled clues? That's the heart of the matter
• Central part of thing - ask us, uncle
• Uncle Dud and Uncle Sam essential to cell
• Core group
• In the sun the clue is in the central part
• Students accept poor clue for 'Core'
• Central part of a cell
• Integral part of anagrammatical clue in up-and-coming paper
• Poor clue in paper rejected, but it's a basis for development
• Part of atom