Depending on the type of clock, contact microphones 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.
Alternatively try to find a spot on the case as close to the movement as possible.
Another option which provides direct contact with the movement 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.
Peeling off the rubber cushioning provides better contact and can help with very quiet movements.