Related Clues:

• Sign of anxiety, perhaps
• Spasm geneticist conceals
• First "O" in a game
• First of three X's
• ___ Tacs
• First syllable of a kids' menu game
• Word one of three involving X's and O's
• Twitch irregularly — or regularly — with this
• Bit of a jerk
• Involuntary jerk, e.g
• Little bit of personality
• Sort of movement some solicitor set up
• With 50 and 55 Across, the ''central'' theme
• Wrinkling one's nose, perhaps
• What the odd letters in "twitch" spell