What kind of clearance do I need for 42a?

You need a secret clearance to hold the MOS. They'll look at your background for criminal charges and your credit to make sure you aren't a financial risk.

Secret at a minimum.

They look at a lot of things, especially financial and criminal. Domestic violence? Typically a career ender. That said, be aware that the MIL and many on the Civil Service side who have positions that don't require a clearance are not off the hook. The concept is known as "maintaining eligibility for a clearance". So if you muck it up such that potential future hit would show up on a background investigation, action can be taken then for separation. We don't keep people around who won't pass muster on classified stuff. I've administratively separated both MIL and CS. Fairly easy mechanical process. Even easier is when the person refuses to provide information "because they have rights". That one took about two days.