Law and custom?

There was no law against running for president as many terms as you wanted until FDR won a third and fourth term. At that point, the custom of only running two terms was permanently enshrined into law.

Retired intelligence officials customarily retain their security clearances unless there are reasons for them not to. There is no law insisting on their right. As former presidents are often diplomatically handy to keep around, it makes sense to retain their security clearance. Much of the behind-the-scenes negotiations with North Korea were accomplished by Clinton and Clinton's officials well into Obama's second term, for example.

However, there's nothing that says they have to keep them. Trump yanked John Brennan's security clearance over a Twitter slapfight. Pretty sure that was just a straight-up executive order, no congressional oversight necessary. And considering how angry Brennan is most of the time, I doubt he would have let it stand if he had any recourse.

Should we ever make it out of this alive, I suspect that a lot of the customs we've banked on to keep our executive branch in order will become laws, much like we gained the 22nd Amendment shortly after FDR passed. One of The Week's columnists observed that so much of our governance is dictated by custom, not law, and that Trump's entire power is his utter disregard for custom. However, it would be the mildest break of custom, and no violation of law, to yank the security clearances of every single person associated with this administration the minute his successor's hand leaves the bible on Inauguration Day.