Is this something you can apply for prior to getting a job?
That said, it stays with you afterwards, so you can take another job that requires clearance at a different organization/company (provided you have the right level of clearance for the job). I think it expires after some time.
A lot of defense stuff falls in the latter category in which case you can start by needing to hire some sales engineers with security clearances who meet with the defense customers -- mostly contractors themselves. The devs don't necessarily need clearance.
If you are making something sensitive -- like a new nuke -- then you will have trouble going the start up route. IN-Q-Tel, DARPA, these programs were designed to fill the gaps.
But if you are making an AI for drones, for example, or some new polymer that has good materials properties that would be useful on a tank, then you can ease into it as these have multiple applications and you just need your SEs to meet with the defense people and say "hey, we have this cool tech, are you interested in it?" and then if they are interested they will work with you on clearances by helping sponsor key employees and letting you know what their requirements are and how you can meet them.
In many cases an acquisition by said contractor is your exit.