Related Clues:

• It comes as a shock
• Involuntary contraction
• First O of O-O-O
• First correct Hollywood Square
• First X?
• First X or O?
• Convulsion
• Part of three-in-a-row
• Nervous habit
• Sudden spasm
• Quiver
• Quaver
• Certain X or O
• Half a mint?
• 4.0, maybe