Okay, ClearPass 6.7 will store only the private key file associated to the latest CSR generated in the server.
And you need to choose the option "Upload Certificate and Use Saved Private Key" when importing the signed cert.
The server will prompt for private key when the stored private key is not matching the signed cert.
In such case, you would need to generate a new CSR again and get it re-signed and import.
Always use the latest CSR from the server. The server will not keep the private keys associated to the old/earlier CSRs.