SCOTT@orcl_11g> Create table test(A varchar2(50),B NUMBER)
  2  /

Table created.

SCOTT@orcl_11g> insert all
  2  into test (a) values ('2*3')
  3  into test (a) values ('2*4*5')
  4  into test (a) values ('4*5')
  5  select * from dual
  6  /

3 rows created.

SCOTT@orcl_11g> select * from test
  2  /

A                                                           B
-------------------------------------------------- ----------
2*3
2*4*5
4*5

SCOTT@orcl_11g> create or replace function multiply
  2    (p_a in	 test.a%type)
  3    return	 number
  4  as
  5    v_result  number;
  6  begin
  7    execute immediate 'select ' || p_a || ' from dual' into v_result;
  8    return v_result;
  9  end multiply;
 10  /

Function created.

SCOTT@orcl_11g> show errors
No errors.
SCOTT@orcl_11g> select a, multiply (a) from test
  2  /

A                                                  MULTIPLY(A)
-------------------------------------------------- -----------
2*3                                                          6
2*4*5                                                       40
4*5                                                         20

SCOTT@orcl_11g> 

select  a,
        dbms_aw.eval_number(a) b
  from  test
/

A                                                           B
-------------------------------------------------- ----------
2*3                                                         6
2*4*5                                                      40
4*5                                                        20

SQL> 

select  a,
        xmlcast(xmlquery(a returning content) as number) b
  from  test
/

A                                                           B
-------------------------------------------------- ----------
2*3                                                         6
2*4*5                                                      40
4*5                                                        20

SQL> 

select  a,
        extractvalue(dbms_xmlgen.getxmltype('select ' || a || ' x from dual'),'/ROWSET/ROW/X') b
  from  test
/

A                                                  B
-------------------------------------------------- -----
2*3                                                6
2*4*5                                              40
4*5                                                20

SQL>