Primary key does not allow NULL, where in clustered index allow NULLs. Clustered index without primary key creates an Unique, PRIMARY KEY NONCLUSTERED index on the table

Yes, you can create. The main criteria is that the column values should be unique and not null. Indexing improves the performance in case of huge data and has to be mandatory for quick retrieval of data.

Yes, You can but make sure value should be unique and not nul

http://blog.ithubcity.com/details/sql-server/sql-server/8/154/can-we-create-clustered-indexes-without-primary-key-and-none-clustered-index-with-primary-key?

Primary key is a constraint and clustered index is an index so, logically they are both different entities in RDBMS. As a matter of fact, this is possible in one condition, when clustered index is created before the primary key on the table otherwise primary key will by default create the clustered index with it and only one clustered index is allowed per table.

No

yes