basically if you are in a 'need-to-know' position you can get clearance. this can come from working at the cia or nsa or fbi or any of our nations 13 intelligence agencies or from working for a dod contractor, examples are boeing, lockheed martin, raytheon, etc. also i guess you could get clearance if you work for other branches of the government, as in department of energy, nasa, a member of the us senate, etc.
i refer to 'need-to-know' as defined in Clinton's Executive Order 12968 August 2, 1995 Sec 1.1h