No syntax for Nonclustered index in MySQL?
InnoDB:  The PRIMARY KEY is UNIQUE and clustered -- always.
Secondary keys are non-clustered.  They have the PRIMARY KEY (not a rownum or rowid) in the leaf node of the secondary key index.

MyISAM:  Keys are non-clustered -- always.  The leaf nodes of indexes have a pointer (think: byte offset) into the data file to get from the index to the data.  ALTER TABLE tbl ORDER BY ...; will reorder the "data", thereby temporarily give you clustering on whatever you ORDERed BY.

MySQL decided those were "good enough".  I have only very rarely found a need for something else.