The Clock item is an item that can have one of two functions: it will either increase the time limit for the round or stop time altogether. It's mostly found in single player modes.
By stopping time it can make clearing the round very easy for the player. All onscreen enemies will freeze and destroying them becomes a simple task. The player should be careful not to overestimate the time of the clock and run into an enemy on accident.
By increasing the time limit it can make for a higher score when clearing the round. It can be advantageous to beginners who need the additional time to clear the round.