Related Clues:

• Piano trio
• They're meant to be engaged
• Stick selections
• They may get engaged
• Rides a bike
• Foot levers
• Two on a tricycle
• Parts of some cycles
• They may be engaged
• First, second, third and reverse
• First, second et al.
• What a shift shifts
• P, R, D, N, 1 and 2
• Clockwork, mostly
• Transmission choices