He does not issue security clearances, he has no authority to revoke them. As far as I know, the only reason to take away someone's clearances is if they are no longer needed for their job, or if the holder has committed a crime or violated classification rules or procedures.

Brennan, and the others he's threatened with revocation, have clearances because they are on call as consultants and to advise officials in security agencies due to their long past experience and time on the job. They receive briefings so their expertise is up to date and is available when needed. Revoking their clearance because they have offended the President is akin to shutting down a newspaper because its editorial policy offends the President. Or maybe he's going to start revoking people's college degrees, or attorney memberships in the Bar, or medical diplomas. What's next, shutting down businesses that don't contribute to his campaigns, or going after late night talk show hosts? Maybe he'll fire government employees just hours before they are eligible for their retirement pensions. That will get the message across.

Of course, like all his other outrages, this was done to test the waters, to see if he could get away with it. If he gets any resistance he will back off from the other names on his shit list. In the meantime, we are all distracted from Manafort and Amarosa.

This is much, much worse than saying the N-word. He's silencing critics by attacking their livelihoods.