Gabbard's defamation complaint centers around a statement made last year on a podcast in which Clinton was discussing Russian interference in the upcoming Presidential election. Clinton said:

"They're (the Russians) also going to do third party. I'm not making any predictions, but I think they've got their eye on somebody who's currently in the Democratic primary and are grooming her to be the third-party candidate. She's the favorite of the Russians, they have a bunch of sites and bots and other ways of supporting her so far, and that's assuming Jill Stein will give it up, which she might not, because she's also a Russian asset. Yeah, she's a Russian asset, I mean totally. They know they can't win without a third-party candidate. So I don’t know who it’s going to be, but I will guarantee you they will have a vigorous third-party challenge in the key states that they most needed."

While the reasonableness of Clinton's case that "asset" here just means something valuable, Clinton did not explicitly state that there is a relationship between Gabbard and Russia. 

It's likely that this statement is protected speech speculating on how a candidate might be perceived by other governments. This type of speech is common in every election.

Especially so since Tulsi is a sitting member of Congress and the definition of a public figure.

Over the years, the courts have established some tests for defamation. The statement must be published, false and "injurious" (proven damage to reputation). If the defamed person is a public figure (like a politician or celebrity), the libelous statement must be made with "actual malice," meaning it wasn't an "honest mistake," but a conscious decision to publish a lie. This is a very high bar. 

In a 1964 case, New York Times Co. v. Sullivan , the Supreme Court ruled that the First Amendment places strong limitations on defamation claims, especially those suits filed by public figures.

Following the rule of the New York Times Co. v. Sullivan case, when a statement concerns a public official, such as Tulsi Gabbard, it is not enough to show that the statement is false for the speaker to be liable. "Instead, the target of the statement must show that it was made with knowledge of or reckless disregard for its falsity."

Because Gabbard is a public figure, the statement was speculating on how a candidate might be perceived by other governments, and it will be difficult to prove Clinton made a false statement "with knowledge of or reckless disregard for its falsity", the court is unlikely to side with Gabbard's defamation claims.