"Security clearance" is usually the termed used for the process by which an employee, or prospective employee, of an commercial organisation or Government department is cleared to handle confidential or sensitive information. It would enquire into the individual's background, possible criminal record, and might even look into their private lives. The process of security clearance, and its rigorousness, varies from organisation to organisation, and so far as I am aware there is no general method in the UK of becoming "security cleared" prior to actually applying for employment.