Possible Clues:

• Established outlet only closed after nag departed? That's too late
• Fit into Dot's building, otherwise it can stop the horses bolting
• Lad presumably has a job opening it for firm on the way out
• Crosses back carrying board for part of farm building
• Bolted after the horse -- it's too late!
• What's bolted as ordered by men, too late?
• It's often locked after the horse has bolted
• That's to keep a horse in