SQL & PL/SQL

Problem is as follows: I need a concatenated list of names somewhere in my query. This is easy enough to accomplish with LISTAGG, but the result may become longer than 4000 characters, so I decided to use XMLAGG. Query goes like this:

SELECT 
  id_flurstueck, 
  REGEXP_REPLACE(RTRIM(EXTRACT(XMLAGG(XMLELEMENT(e, bezeichner || '; ') ORDER BY bezeichner), '/E/text()').getclobval(), '; '), 
                 '([^;]*)(; \1)+($|;)', 
                 '\1\3') AS eigentuemer 
FROM 
  mv_alkis_eigentuemer 
GROUP BY 
  id_flurstueck ;

mv_alkis_eigentuemer is a materialized view and the query gives me the expected answer. Slow - but working.

But I need those names in a longer query, and so I join on that SELECT.

SELECT 
  ae.eigentuemer, 
  manyOtherAttributes 
FROM 
  myTable t LEFT JOIN ( 
                        SELECT 
                          id_flurstueck, 
                          REGEXP_REPLACE(RTRIM(EXTRACT(XMLAGG(XMLELEMENT(e, bezeichner || '; ') ORDER BY bezeichner), '/E/text()').getclobval(), '; '), 
                          '([^;]*)(; \1)+($|;)', 
                          '\1\3') AS eigentuemer 
                        FROM 
                          mv_alkis_eigentuemer 
                        GROUP BY 
                          id_flurstueck 
                       ) ae 
            ON         t.id_flurstueck = ae.id_flurstueck ;

Unfortunately this results in

ORA-00932: inconsistent datatypes: - expected, CLOB received

Same result if I use the query inline as a correlated subquery.

Any ideas, how to overcome this?

Database is Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production.

Thanks in advance, Bernd