Great rundown from WTOP (Washington’s Top News) in part):
FYI to this story and the question raised from the Washington Post: What does Brennan or other former officials he need a security clearance for anyway?
He no longer works for the government. Why does he need access to government secrets?
A: When senior officials leave government — on good terms there has been an informal standard of continuing to grant clearance for the rest of their careers. Those clearances go through a review process every five years.
More examples and answers to a lot of questions people are asking at the linked story about this issue — a good read — check it out.