Based on the code snippet you have posted, I think the actual result is what you could expect based on the read attribute of the ColumnTransformer annotation. Using this annotation you can define a custom SQL expression to read the value from and write a value to a column. And you tell Hibernate to use the to_clob function on the PRESENT_APPROVERS column. You don't use the appropriate function to extract the value from the EMAIL node.
I have no knowledge about the Oracle database, but it seems the function you are looking for is EXTRACTVALUE. So I would expect that you need to change your code like this to get the expected result