The standard advice I've heard is: Work for defense companies to learn the technology, and then leave to transfer this technology toward peaceful (and often lucrative) applications. Swords into Ploughshares.
I know of one aerospace company that occasionally has its rockets explode on the launchpad because they use battery designs from the 1970s that are known to explode with fairly high probability. They could easily get much cheaper much more reliable batteries with more modern designs but they don't because the exploding batteries "have flight heritage". This particular aerospace company is considered agile and nimble and innovative compared to its competitors.
I don't work in defence. That is just what I've heard from people who do as one of the reasons why it's hard to "rotate in, rotate out."
Huh? None of my friends with clearances seem to have that problem. Do they have the "wrong" clearances?
Doing secure work means you can't take any confidential information with you when you leave, and it might slightly limit what you can talk about, but it doesn't mean you can't say anything about the work you did. You can still talk about your role and skillset, usually about the languages and technologies (and COTS products) used.
Many government IT projects mandate security-cleared developers because the systems touch classified information, but that doesn't mean that the systems themselves (their existence, basic design, etc.) are classified. There's really no difference between having a project like that on your resume, and any other commercial-sector project that's internal use only.
If you're working or considering working on a clearance-required project, it's not unreasonable to ask to talk with the security officer beforehand to clear up exactly what you'll be able to put on your resume and discuss after the fact ... and price yourself accordingly.