Hybrid Synchronous / Asynchronous Replication?
It's simpler that you may expect.

Use one of the nodes in the Galera/PXC cluster as the master for the async replica.  

The node acting as master for the slave needs the following additional configuration parameters for this to work:

log_bin
log_slave_updates
binlog_format = ROW


Connect an asynchronous MySQL/Percona/MariaDB slave to it using a normal asynchronous master/slave replication configuration.

That should be about it.  The slave ideally will be the same flavor and release series as the cluster nodes, either MariaDB or Percona on both ends, though a MariaDB slave is pretty much always a safe bet, regardless of the master.  In no case should the slave run an older version, so when you do upgrades, always upgrade the slave first.  You can also subtend additional slaves from the first one, or connect a second slave to a different cluster node as its master.

Bonus: enable slave_compressed_protocol on the slave so that the data streamed from the master is compressed on the wire.