Is it true that clumsiness is a disorder or a behavior?

Can be a symptom. Clumsiness could be a symptom of a neurological disorder. If a person has a medical condition which is causing damage to the cerebellum, the part of the brain that controls coordination and balance, then the person could be "clumsy" as a result of the condition. Most of the time, a person described as clumsy probably does not have a neurological condition, however.