Hi,
It doesn't appear you can set the on screen clock to display seconds. If you quickly need to obtain HH:MM:SS just compose an email and type LT in the body of the email, when you press space it will provide the required information. As a more functional workaround you can install WorldMate Live (Free Download:
https://www.worldmatelive.com/) and it has a great World Time application which displays seconds.
Thanks,
Gary