If you still has your former passport and your country does allow dual nationals/citizens and you did not denounce your former nationality, you will not able to get a top secret clearance. Even you had done all that, many people including natural born citizens still won't able to get top secret clearance because of the complexity of its background checks and requirements. You may albe to get a lower security clearance which should be easy to obtain as long as you have a clean background and met the requirements but dual nationals will still be an issue and could turn into a roadblock for obtaining security clearance. I have a friend that had served in the US navy in the E3 rank for nearly 6 years but still only able to get a secret clearance when he got discharged and worked at China Lake.
