Possible Clues:

• Epiphany
• Puzzle solver's breakthrough, e.g
• Lighting event?
• *Light-bulb-over-the-head instance
• Puzzle solver's epiphany
• Instant when things click
• Instant of insight
• Sudden insight ... with a hint to 17-, 24-, 40- and 50-Across
• An actor warning cast finally, when he twigs
• Mental breakthrough