Possible Clues:

• When Sally sings "Mein Herr" in "Cabaret"
• Start of some entertainment
• When the nude scene occurs in 10-Across
• Musical starter
• Theatrical kick-off
• When Caesar is told to beware
• Stage-play starter
• When Hamlet sees the ghost
• Start of 'Hamlet'
• When the Wicked Witch dies, in 'Wicked'