Clearing means processing unsettled transactions. Specifically - all the money transfers between the banks, in this case.
Clearing Bank for RMB business means that all RMB transactions will be cleared through that specific bank. If bank A in Hong Kong gets a check drawn on Bank B in Hong Kong, and the check is in RMB - A will go to the BoC with the check and will get the money, and BoC will take the money from B. That obviously requires both A and B have accounts with BoC.
"Sole" clearing house means there's only one. I.e.: in our example, A and B cannot settle the check through C where they both happen to have accounts, or directly with each other. They MUST utilize the services of BoC.