Possible Clues:

• Its movement can cause sudden drop in this species of spider
• Gin before opening may let an actor down
• Thing fallen through in a floor
• Concealed entrance charge after time -- party has alternative
• It may be under the rug
• It might offer a sudden opening to a player
• Cross section rejected means of escape
• Opening cross section up
• Cross section raised to form emergency exit
• Theatrical entrance from lifting crosspiece