This is a welcome development. Trump's lawyers will argue that this is a matter of "public interest," so the standard for a defamation action to proceed is that Trump spoke with "actual malice," i.e., without any regard for the truth of what he said. Of course, this is tricky, because if he would know whether the allegations she made were true, and if they are true and he still called her a liar, then it certainly looks like actual malice! (A court might also simply treat the plaintiff as an ordinary citizen, so it will simply suffice if the statement that she is a "liar" is false--but in this context, for the reason noted, this may no matter.)
UPDATE: Some more details, including the complaint, here. An ironic sidenote: Trump apologist and spokesperson Kellyanne Conway is married to the lawyer (George Conway) who briefed the Supreme Court case that held that a sitting President (in that case Bill Clinton) could be a defendant in a civil suit. Mr. Conway is mentioned as a possible candidate for Trump's Solicitor General!