How to generate JSON in Oracle for a CLOB that is > 32k (e.g. 60,000 characters)?

In answer to this question:

3 approach solve my problem but i don't want to run for loop . Is there is any solution in oracle to handle this .

Strings can be concatenated without looping by using Oracle's LISTAGG function:

SELECT '{"employees":[' || LISTAGG('{"employee_id":' || to_char(employee_id)
                      || ',"data_clob":"' || data_clob || '"}', ',')
              WITHIN GROUP (ORDER BY employee_id) || ']}' AS json
FROM tablename;

However, as you've pointed out in the comments, LISTAGG has a limit of 4000 characters. The following is more complex/fiddly but should cope beyond this limit:

SELECT '{"employees":[' || dbms_xmlgen.convert(
         RTRIM(XMLAGG(XMLELEMENT(E,'{"employee_id":' || to_char(employee_id)
                                 || ',"data_clob":"' || data_clob || '"}',',')
                      .EXTRACT('//text()') ORDER BY employee_id).GetClobVal(),',')
       , 1) || ']}' AS json
FROM tablename;

XMLAGG handles CLOBs but the EXTRACT function has the side-effect of escaping certain characters (e.g. from " to "). The query above converts these back (e.g. from " to ") using the dbms_xmlgen.convert function - see this answer for further details.

SQL Fiddle demo: http://sqlfiddle.com/#!4/5b295/40