Clamp meters, those that sit on top of cables, don't give voltage as far as I've seen, although I've never actually used one. They work by coupling a coil around the cable to induce a current which is then converted and measured. Some clamp meters do have voltage capability, these variants tned to have some probes which you then must shove into the socket to measure voltage.
Clamp meters also don't work on multicore mains cables as the live and neutral cancel each other out. So unless you can separate the two conductors (don't do this unless you know what you are doing) you'll get a zero result.
Get a plug in watt meter thing. Safer, reads voltage, current and counts kWh.
If you wish to measure low voltage DC then get a multimeter.