Possible Clues:

• Opening in a floor
• Opening in a floor or ceiling
• Prop that enabled Houdini to 'walk through' a brick wall
• Review cross section making exit from stage
• Cross piece knocked over, revealing secret entrance
• Feature of a magician's stage
• Concealed exit from port road undergoing repairs
• Might such an entrance catch one unawares?
• Opening in the floor
• Magician's way out, maybe