I'm afraid that you can't just get security clearance, what tends to happen is that you get the job and then get security cleared whilst working. It's a chicken and egg situation, can't get it without the job and can't get the job without it, if you've not been in the forces it's a matter of luck. Asking for security clearance will save the employer a lot of time and expense but they may not find a security cleared person, so don't give up.