how to reduce current without changing voltage?
This is a duplicate of the many questions on this site where people are not understanding Ohm's law.

The important thing to realise with your clock, and with any electronic device, they will draw the current they are designed to draw if given the voltage they are supposed to be given. Plug the clock into the 5V, 2A source, and it will draw significantly less than 500mA. All will be fine.