If you have something — such as a letter, an email or a contract — in which you've been commissioned by a specific publication to write a specific story, there's nothing wrong with telling people, "I'm Johnny Smith, and I'm working on a story for Lawn Darts Illustrated."
In the interest of disclosure, if you're asked, you should acknowledge that you're a freelance writer. But you can certainly add that you were commissioned by the publication to write the story.
If you have been commissioned, stating up front that you're a freelancer isn't really necessary. At best, it doesn't matter to most people, or they won't understand the difference. At worst, it will scare off media-savvy sources who might think you're just some schmuck writing the story on spec and trying to sell it to someone, anyone.
If you actually are some schmuck writing a story on spec, you can say you're a professional writer (reporter, journalist, author or some other noun that strikes your fancy) who's working on a story that you intend to publish in the future. If pressed, you should acknowledge your story doesn't have a home yet, but you can add that your work has appeared in such outlets as A, B and C in the past (or that you've worked for those organizations, if you were on staff).
In all cases, don't mislead anyone.
Unless you're Jim Rockford pretending to work for Guaranteed Insurance & Casualty, trying to get Angel out of another jam.