Can I use a Clob as a table key (from Java)?
The simple version would be to generate a hash from the CLOB and use this as a key. This key will fit into the allowed key width for your engine (900 SQL Server, 767 InnoDB, 1000 MyISAM etc).

The hash can be generated by the engine as a computed column, trigger, or by some ETL process, or by the application

There is a faint chance of collision (birthday problem) but this would only matter for many records and poor choice of hashing algorithm