Possible Clues:

• Crowning achievement
• Crowning event
• Opens act with flourish: a crowning achievement?
• Part of coping with opencast working
• What's put on wall once past courses
• Piece on top of wall, etc
• Space not properly allocated for top bit of structure
• Gangster's covering street, in top part of building
• It forms the top of a wall
• First on stage, not turning to don cloak and topper