How do you repair a stopped clock in Pokemon Sapphire?

You can't repair a stopped clock in Pokemon Sapphire. There isn't a mechanic in the game that allows you to repair objects like that.

The clock is a decorative element in the game, and its purpose is to show the current in-game time. It is not a functional item that can be repaired.

If you're experiencing a bug related to the clock's display, it's likely a glitch, and restarting the game may help.