You have to escape the single-quotes
SELECT STRING_AGGREGATE('select ename||'' Job is ''||Job from emp') ENAMES FROM DUAL;
You can try out out how you pass the string to the function like this
SELECT 'select ename||'' Job is ''||Job from emp' FROM DUAL;
which gives you
select ename||' Job is '||Job from emp
See the demo: http://sqlfiddle.com/#!2/d41d8/23283
(By the way. There is a new feature LISTAGG
since Oracle 11g which you may also want to have a look at.)