Mains voltage is an AC signal so all you have to do is get the voltage down to something save and put it through a Schmidt trigger and you'll get a nice 50 Hz block signal, which should work fine as clock signal.
Mains voltage is an AC signal so all you have to do is get the voltage down to something save and put it through a Schmidt trigger and you'll get a nice 50 Hz block signal, which should work fine as clock signal.