There are special conditions where you can keep your clearance after leaving government employment but none that I am aware of where a private citizen can get a clearance just in case they might need them.
And just because someone keeps there clearance, say someone like Clapper with a TS/SCI, doesn’t mean they can go look at someone’s SCI material. There is still a need to know provision.
The highest codeword materials are strictly compartmentalized.
I had a TS/SCI clearance up until 1982 and worked at HQ/SAC. I was flabbergasted when they disclosed the stealth fighter and said it first flew in 1981. I knew absolutely nothing about the R&D effort.
One reason, I was against the post 911 changes is that I think we made too many changes and information sharing between agencies (even though, supposedly, still secure) over an awful event but one should make drastic changes to a single event, especially when the "real" threat is not a bunch of 3rd world Muslims (and some with lots of money and 1st world status) but large state actors with long history of infiltration and expertise. Yes, Muslim terrorism is a problem but to change so much based on that versus folks like Russia, China, and others (even some of our "friends.")
There should be sharing but, also, independent methods and operations are needed to cross-verify, develop own sources or ideas, protect civil liberties, protect sources and projects, allow for "plausible deniability," and such. Remember the INSLAW and PROMIS stuff back when? I worry that a lot of the information sharing, out-sourcing, and agency shake-ups, and "cloud" idea have compromised our security.