The suit, if there were one, would be against Ed Klein for writing something damaging to another.
Source is irrelevant. Whether it is claimed to be fact or fiction is irrelevant.
The only issue is damage done...if such damage is not grounded in fact.
The rules for defamation of character vary from state to state but there are standard elements:
The defaming matter should be published,
It must be false,
it must be injurious, and
It must be unprivileged (i.e.not made in court).
Yes, but Hillary is a public figure and subject to rumor and fact. I’m saying fictitious sources won’t be suing any time soon. :)