Possible Clues:

• Core group of students tackling tricky clue
• Core group of students to welcome around the French university
• Our setter flipping about labyrinthine clue's central part
• Central mass or kernel
• Core (of atom)
• Positively charged central core of an atom
• Sun flips about changed clue for atomic base
• Core muscle, with miles gone, stretched by character in Marathon
• Benefactor promoting new American centre
• Tricky Sun clue for central part