Possible Clues:

• Brexit target org
• Continental org
• 27-member gp
• What was exited in Brexit
• Org. whose flag has 12 stars
• Grp. whose flag has 12 stars
• What's exited in Brexit
• What's exited in Brexit, for short
• 2012 Nobel Peace Prize recipient, informally
• What Britain voted to Brexit from, for short