En el * tienes dos columnas que son username . Necesitas calificar uno o ambos. Ejemplo a continuación:
SELECT
customers.id,customers.name,customers.username,customers.phone,customers.email,radcheck.value as password
FROM customers
RIGHT JOIN radcheck ON customers.username = radcheck.username
LEFT JOIN (
SELECT radrecent.username, latest FROM radacct INNER JOIN (
--^^^^^^^^^
SELECT username,MAX(acctupdatetime) AS latest FROM radacct GROUP BY username
) as radrecent
ON radacct.username = radrecent.username
AND radacct.acctupdatetime = radrecent.latest
) as radlatest
ON customers.username = radlatest.username
WHERE radcheck.attribute = 'Cleartext-Password'
