Many mantel clocks have a door on the back of the clock. Tambors and many cottage clocks are typical examples. Opening this door reveals the clockworks including the pendulum. Most people are inclined to start the clock by pushing the pendulum, closing the door, and then turning the clock around. This method can be cumbersome and not always successful. There is a better way.
Set the clock on the shelf or mantel on which it will be used. Make sure it is level. Then lift either the left or right side of the clock about a half inch and then immediately set it back down. When done smoothly, this simple motion is enough to start the pendulum swinging. Your clock is now running!