CREATE table dept  (department_id, department_name) AS
SELECT 1, 'IT' FROM DUAL UNION ALL
SELECT 2, 'SALES'  FROM DUAL;

CREATE TABLE employees (employee_id, manager_id, first_name, last_name, department_id,
serial_number) AS
SELECT 1, NULL, 'Alice', 'Abbot', 1, 'D123' FROM DUAL UNION ALL
SELECT 2, 1, 'Beryl', 'Baron',1,'D124' FROM DUAL UNION ALL
SELECT 3, 1, 'Carol', 'Chase',1,'A1424' FROM DUAL UNION ALL
SELECT 4, 2, 'Debra', 'Doris',1, 'A1425' FROM DUAL UNION ALL
SELECT 5, 3, 'Emily', 'Evans',2,'C1725' FROM DUAL UNION ALL
SELECT 6, 3, 'Fiona', 'Frank',1,'C1726' FROM DUAL UNION ALL
SELECT 7, 6, 'Gemma', 'Grace',2,'C1727' FROM DUAL;

 select  d.department_name,          listagg(e.last_name,  
chr(10)) within group (order by e.last_name) 
employees
    from employees e join dept d on e.department_id = d.department_id
    group by 
    d.department_name;

DEPARTMENT_NAME    EMPLOYEES
IT    Abbot
            Baron
            Chase
            Doris
            Frank

SALES      Evans
             Grace

SQL> with
  2    dept  (department_id, department_name) AS (
  3  SELECT 1, 'IT' FROM DUAL UNION ALL
  4  SELECT 2, 'SALES'  FROM DUAL ),
  5    employees (employee_id, manager_id, first_name, last_name, department_id, serial_number) AS (
  6  SELECT 1, NULL, 'Alice', 'Abbot', 1, 'D123' FROM DUAL UNION ALL
  7  SELECT 2, 1, 'Beryl', 'Baron',1,'D124' FROM DUAL UNION ALL
  8  SELECT 3, 1, 'Carol', 'Chase',1,'A1424' FROM DUAL UNION ALL
  9  SELECT 4, 2, 'Debra', 'Doris',1, 'A1425' FROM DUAL UNION ALL
 10  SELECT 5, 3, 'Emily', 'Evans',2,'C1725' FROM DUAL UNION ALL
 11  SELECT 6, 3, 'Fiona', 'Frank',1,'C1726' FROM DUAL UNION ALL
 12  SELECT 7, 6, 'Gemma', 'Grace',2,'C1727' FROM DUAL )
 13   select  d.department_name,
 14           listagg(e.last_name||' '||e.first_name, chr(10))
 15             within group (order by e.last_name)
 16           employees
 17  from employees e join dept d on e.department_id = d.department_id
 18  group by d.department_name
 19  /
DEPAR EMPLOYEES
----- ----------------------------------------------------------------------------------------------
IT    Abbot Alice
      Baron Beryl
      Chase Carol
      Doris Debra
      Frank Fiona
SALES Evans Emily
      Grace Gemma

2 rows selected.

SQL>   SELECT d.department_name,
  2           LISTAGG (e.last_name || ' ' || e.first_name, CHR (10))                       --> added here ...
  3              WITHIN GROUP (ORDER BY e.last_name, e.first_name) employees               --> ... and here
  4      FROM employees e JOIN dept d ON e.department_id = d.department_id
  5  GROUP BY d.department_name;

DEPAR EMPLOYEES
----- ------------------------------
IT    Abbot Alice
      Baron Beryl
      Chase Carol
      Doris Debra
      Frank Fiona

SALES Evans Emily
      Grace Gemma


SQL>