CREATE TABLE table_name (value) AS
SELECT ' mon tue wed thu ' FROM DUAL

SELECT TRIM(BOTH ',' FROM REGEXP_REPLACE(value, '\s+', ',')) AS replaced_value
FROM   table_name;

SQL> CREATE OR REPLACE FUNCTION f_space_to_comma (par_string IN VARCHAR2)
  2     RETURN VARCHAR2
  3  IS
  4  BEGIN
  5     RETURN TRIM (BOTH ',' FROM REGEXP_REPLACE (par_string, '\s+', ','));
  6  END;
  7  /

Function created.

SQL> select f_space_to_comma(' mon tue wed thu ') result
  2  from dual;

RESULT
--------------------------------------------------------------------------------
mon,tue,wed,thu

SQL> CREATE TABLE table_name (value) AS
  2  SELECT ' mon tue wed thu ' FROM DUAL;

Table created.

SQL> select f_space_to_comma(value) result
  2  from table_name;

RESULT
--------------------------------------------------------------------------------
mon,tue,wed,thu

SQL>