What kind of security clearance are you talking about? Enhanced/secret/top secrect security check? For enhanced security check they will verify the information you provided for the past five years. It includes criminal record/address/reference check. it will take very long time for new immigrant, especially people from China, to pass the check. The Canadian government need the co-operation from the Chinese authority to verify some information. For some reasons it is very time consuming. It may take 1-2 years to get through the process, which makes it impracitical for an new immigrant to get a job when such a check is required for the positon