Possible Clues:

• Airline service: con or easy target?
• But for poor Don King beside 2, contender must hit it!
• No point in locking it after the horse has bolted
• Ban donor in a way from opening on farm
• 'Don't close the -- after the horse is gone'
• Easy target making one flap
• When the cows come home, they go through it
• Ban furious donor for missing the proverbial target
• You'd probably miss this proverbial pub opening after the end of autumn
• One can't miss this farm entrance