Considered unethical, to call the clock is to ask the floor manager to put a player to on the clock for a decision. A player may call the clock on an opponent if he/she is taking an inordinate amount of time over a decision or to put pressure on them. The amount of time you get will vary from casino to casino. Be careful not to take too long over decisions or you may get called the clock on a lot.
Check out the video below where Daniel Negreanu is put on the clock after taking less than a minute. Clearly the player wanted to Daniel to fold but did he?