SQL> select reverse('WELCOMETOPAKISTAN') from dual;
REVERSE('WELCOMET
-----------------
NATSIKAPOTEMOCLEW

1 row selected.

SQL> declare
  2  a number;
  3  begin
  4  select length(a1) into a from dual;
  5  select substr(:t,a,-a) from dual;
  6  end;
  7  /
SP2-0552: Bind variable "T" not declared.

select length(a1) into a from dual

 a := length(a1);

SQL> ed
Wrote file afiedt.buf

  1  create or replace procedure mine_reverse(a in varchar2) as
  2  another varchar2(20);
  3  begin
  4  for d in 1..length(a) loop
  5  another:=another||substr(a,-d,1);
  6  end loop;
  7  dbms_output.put_line('here is reverse  '||another);
  8* end;
SQL> /

Procedure created.

SQL> execute mine_reverse('Pointers');
here is  sretnioP

PL/SQL procedure successfully completed.

SQL> set serveroutput on;
SQL> execute mine_reverse('pointers');
here is reverse  sretniop

PL/SQL procedure successfully completed.

with src as (select 'Pointers' word from dual)
select regexp_replace(xmlagg(letter),'<[^>]+>',null,1,0)
from (
select xmlelement("X",substr(word,level,1)) letter from src connect by level <= length(word) order by level desc
);

SQL> with
  2    data as ( select 'Pointers' word from dual ),
  3    letters as (
  4      select substr(word,length(word)-level+1,1) letter, level rank, length(word) last
  5      from data
  6      connect by level <= length(word)
  7    )
  8  select replace(sys_connect_by_path(letter,'/'),'/','') "Reverse"
  9  from letters
 10  where level = last
 11  connect by prior rank = rank - 1
 12  start with rank = 1
 13  /
Reverse
-----------------------------------------------------------------
sretnioP

1 row selected.

SQL> with
  2    data as ( select 'Pointers' word, rownum rn from dual )
  3  select r "Reverse"
  4  from data
  5  model
  6    dimension by (rn)
  7    measures (word, cast(null as varchar2(100)) r)
  8    rules
  9    iterate (100) until (substr(word[1],iteration_number+1,1) is null)
 10    ( r[1] = substr(word[1],iteration_number+1,1) || r[1] )
 11  /
Reverse
-------------------------------------------------------------------------
sretnioP

1 row selected.