SC level clearance is a great thing for getting jobs working on government contracts.
Your employer should have a security officer or such like who will send you a whole stack of paperwork to do. This should consist of several forms, including one mainly for your personal details and one for your financial situation.
The first one is simple really, just don't lie on it, because they will know. Also make sure you explain any missing parts in your life. I.e. I had to detail why I lived in four different places in three years (Uni). It also has a brilliant question which asks are you a terrorist or do you know any. It says explicitly no stupid answers, but I have heard stories about people who muck about with that one! It also gets complicated if one or both of your parents were not born here, and they require a lot of evidence to prove you are not a foriegn terrorist! Basically if your parents are from Iran, Iraq, N. Korea etc, forget it!
The second one you will need to list all of your financial obligations. Debts, higher purchance agreements etc. Again no lying here as they will know.
The general gist of it is that they already know all of the answers, they are just making sure you also put down the same stuff, and are not lying about a mountain of gambling debts etc. Also you may be asked to sign the Official Secrets Act. Oh and no jokes around security officers. They have no sense of humour. They are usually quite nice with dealing with people, for example if you get a driving offence or lamp someone in a pub they will try their hardest to make sure you keep your clearance (general rule, if you get stopped for something or done for it, tell them as if you don't, you are likely to find yourself without a job and a bad reference when the do find out).
Clearance lasts for a number of years, but if you leave it 'fades' and you'll have to reapply if you get another SC job over a year after leaving this one. DV, according to my security officer, involves interviews with friends, family, partners etc, and has even been know to have special branch going through your rubbish! (Though I have no idea if this is actually true).
I make it sound all scary and formal, but once you get it, it really opens doors. Just don't take any chances once you have gotten it (e.g. one role I had in a previous job was to escort non SC cleared people around sites, and if I were to let them go to the bog without me, that would be my job and clearance gone).
