Depending on the type of clock, vibration pickups may require some trial and error to determine the best point to clip the sensor. It is usually best to attach the sensor directly to the movement, if you have easy access to it.
Another option which works fine is clipping the sensor to the winding key and inserting the key into the keyhole. Ofcourse this will prevent the clock hands from moving so you have to pick the right time for measurement.
Alternatively try to find a spot on the case as close to the movement as possible.