What proof for Certificate of Clearance (Singapore)?

If you lived in Singapore before applying for Canadian PR, you need a Certificate of Clearance from Singapore Police Force (SPF). The relevant link to obtain this:

It says that they need a proof of why this certificate is required. Does this mean that I cannot apply for this certificate until I get a ITA from Canada? Is there some other document proof I can use to get a certificate of clearance in advance while my PR application is waiting?