create this function:

create or replace function doit(fieldin varchar2, tablein varchar2) return varchar2 is
wholestring varchar2(4000) := '';
nextstring varchar2(4000) := '';
TYPE t_cursor is REF CURSOR;
v_cursor t_cursor;
begin
open v_cursor for 'select '||fieldin||' from '||tablein;
loop
fetch v_cursor into nextstring;
exit when v_cursor%notfound or ((length(wholestring)+length(nextstring)) > 4000);
if wholestring is not null then
wholestring := wholestring||', '||nextstring;
else
wholestring := nextstring;
end if;
end loop;
return wholestring;
end;
/


pass in two strings: the first is a string representing an expression which returns a string, such as a char or varchar2 field name, or to_char(numberfield). The second parameter to be passed in is a string containing the table name (including schema name if appropriate).

For example:

select doit('to_char(id)', 'emp') summary from dual;
select doit('name', 'emp') summary from dual;

OR

set serveroutput on
set heading off
set pagesize 10000
set linesize 122
exec dbms_output.put_line(doit('name', 'emp'));