Clever, Smart and Intelligent are attributes which are used interchangeably and synonymously. They express different levels of formality but essentially they may mean the same.
Smart is being quick-witted or "bright". A smart person is usually someone who is quick in thinking though not necessarily having high level knowledge in a broad base of subjects.
Intelligence is the ability to learn, understand and make judgments or have opinions that are based on reason, and is something which you are born with i.e genetic. A person's intelligence level has to do with how well informed they are, how educated and knowledgeable they have become.
Clever people are born with above-average mental ability using their knowledge creatively. Cleverness is having or showing the ability to learn and understand things quickly and easily.
Clever person know what works, smart person know when it works, and intelligent person knows why it works :)