Depends on the style of clamp.
Some are like Reefer Bill mentioned above, similar to a zip tie.
If not, take a flat screwdriver, key or other hard thin object. If you look at the clam from the side, it probably kind of looks like two alligator mouths facing each other, that lock with the opposite mouth when the clamp is closed.
Just put the screwdriver, key, etc against one of the inside "jaws" and push it gently sideways, and out of the other side of the clamp.
Be careful, they can be stubborn, and you don't want to stab a hose, aquarium glass, or yourself when the clamp lets go.
If that doesn't help, post a pic of the clamp in question.