A treatment is a prose version of your script, and in many cases, treatments can go well beyond one page. It may sound counterintuitive, but to get to a tight one-page treatment, you should first write a treatment where you’re not attempting to limit it to any length at all.
Once you complete your draft – let’s say it’s seven pages – now you can return to it for editing. This process will help you really understand what is at the heart of your story since you’ll be required to delete any part of your treatment that doesn’t reflect the core narrative.
It likely won’t be easy, but as you keep chipping away at the length of your treatment, you can eventually get it to a single page that clearly and succinctly tells the story of your script.