My question involves a background check in the State of: DC

I was arrested at the age of 18 for shoplifting. The charges were dismissed. My academic, professional, financial, and criminal records are otherwise perfect. I'm 22 now.

I was just offered a job at a large consulting firm: the background check asked if "you have ever been arrested for any criminal offenses" - I answered no because I thought a dismissal allowed for you to legally say "no" but after more research I'm beginning to believe this isn't the case. I called the local PD and they couldn't pull anything up.

1) I haven't heard anything from the company, but I'm thinking about contacting the background investigation entity and letting them know that I made an honest mistake on the application. Do you think this will hurt more than it will help? Would this induce a "deeper" investigation?

2) With this arrest record on file, will I be able to get a Secret Clearance from the government?

Thank you for your time.