Possible Clues:

• Saying widely assumed to be true
• Assumption arising from American chopper on Isle of Man
• Statement established as true
• 'Practice makes perfect' or 'Haste makes waste'
• Given in a math class
• A team order that’s generally accepted
• A cross on island making a self-evident statement
• Obvious fact
• "He who laughs last ..." is one
• A team only Mancunians originally saw?