Possible Clues:

• Something many people know although it has not been explicitly divulged
• A known unknown?
• Everyone knows this writer's captivated by awful coterie I won't be part of
• Private matter actually known to many
• Common knowledge
• What everyone knows that they shouldn't know
• Someting poorly hidden
• Traditional oxymoron of MI6, say
• One's generally known to pose with centre fold
• Begins with European island - almost everyone knows it