The Alarm Clock is a power-up that can be purchased and used to make a lot of noise that will distract all enemies in range, enabling the player to slip past them. In Sly 2: Band of Thieves, it is only available for Sly Cooper, while in Sly 3: Honor Among Thieves, it is only available for Bentley.
To use it, the player must press the assigned button (
, 
or 
) and aim where they want Sly or Bentley to throw it, using the arrow that appears. When the button is released, the clock will be thrown and begin ringing upon landing, attracting nearby guards and allowing the player to either escape or eliminate them. It will self-destruct after a limited amount of time.
The gadget is required for "Operation: Canada Games" in Sly 2. It is never required in Sly 3, however.
The TOM gadget serves exactly the same purpose as the Alarm Clock. They are both used to simply distract enemies.