For robbers the clock is an important item. If you are criminal you can get out of the prison with a clock. When your Jailtime is expired you automatically get the clock in your inventory.
If you got a clock in your inventory you have to take it in your hand and do a "rightclick". After right-clicking with the clock you will be ported to the Spawn and you will no longer be cuffed.
You can't buy or sell a clock. The only option to get a clock is waiting for expired Jailtime.