hello Beter,
you can use

dbms_lob.substr( clob_column, for_how_many_bytes, from_which_byte );

and here basic two example :
first by using SQL but note the max length is 4000 characters:

select dbms_lob.substr( clob_col, 4000, 1 ) from Tablename;

second by using PL/SQL max length is 32000 characters :

DECLARE
  var1   LONG;
BEGIN
  FOR i IN (SELECT clob_col FROM tablename)
  LOOP
    var1 := dbms_lob.substr (i.clob_col, 32000, 1);
  END LOOP;
END;

hope this may help.
you can refer to DBMS_LOB from oracle