First you need to determine which model clock you have. If there is only one wire I expect it is a wind-up clock. The wire is for the light.

If it is an electric clock, you will need to do some research to determine what size resistor you need so you get 6 volts to the clock motor. The resistor must have the same resistance as the clock motor so it takes half of the power and voltage.

A fuel gauge resistor might not be the correct choice. It might be the wrong resistance plus it might not be able to handle the amperage that the clock motor requires. The fuel gauge circuit does not draw much current.