Complex sentence: A phobia is a type of anxiety disorder, usually defined as a persistent fear of an object or situation the affected person will go to great lengths to avoid, typically disproportional to the actual danger posed.
Split into simpler sentences: A phobia is a type of anxiety disorder, defined by a persistent fear of an object or situation. The affected person will go to great lengths to avoid the situation or object, typically to a degree greater than the actual danger posed.