The new, enhanced Wordpress management interface in Onyx is a great thing! Unfortunately the important "clone" feature to copy a whole Wordpress installation into a new domain or subdomain requires the PHP proc_open() function. This function must normally be disabled in shared hosting, because it can enable users to execute code on the shell level. For that reason, this great new "clone" feature cannot be used in shared hosting environments 
The problem seems to be that the clone function is run by the subscription user's PHP, not by Plesk PHP engine. If cloning was done by the same PHP instance that Plesk is run under, the disabled PHP commands on the subscription level would not affect the process.
For that reason I suggest to revise the code of the clone function so that it is not affected by disabled PHP commands on subscription level.
The problem seems to be that the clone function is run by the subscription user's PHP, not by Plesk PHP engine. If cloning was done by the same PHP instance that Plesk is run under, the disabled PHP commands on the subscription level would not affect the process.
For that reason I suggest to revise the code of the clone function so that it is not affected by disabled PHP commands on subscription level.