with t as
 (select '1,2,3,4,5' a from dual)
select substr(a,
              decode(level - 1, 0, 0, instr(a, ',', 1, level - 1)) + 1,
              (decode(level,
                      regexp_count(a, ',') + 1,
                      length(a) + 1,
                      instr(a, ',', 1, level))) -
              (decode(level - 1, 0, 0, instr(a, ',', 1, level - 1)) + 1))
  from t
connect by level <= regexp_count(a, ',') + 1