"MySQL Cluster" disambiguation...
.NDB Cluster -- A specialized, mostly in-memory, engine. Not widely used.
.Galera Cluster aka Percona XtraDB Cluster aka PXC aka MariaDB with Galera. -- A very good High Availability solution for MySQL; it goes beyond Replication.
See individual pages on those variants of "Cluster".
For "clustered index" see page(s) on PRIMARY KEY