I had the same conundrum, and through assiduous experiment and searching came up with the following solution: to delete a staff from a double staff (e.g., piano), click the score setup button in the menu at the right-top of the Notion window (3 horizontal lines), then click the cog at the far left of the instrument (e.g., piano); in the little window that appears, click Notation/Tab at the top; below Staff; click on Drum Set (page 11.4 of Web.pdf tutorial) . You get a single staff with only a drum clef sign. To get a treble cleff sign, click on the cog again and then click Standard Notation. I got the treble clef sign this way and assume you can then change it to a bass cleff sign if you wish, by getting the bass clef from the palette at the bottom of the Notion window.
Now if someone could solve my problem of dynamics not changing the volume of an instrument (Mac OS Big Sur, Notion 6.8.1 build 18093)! Notion deservers very high marks in some areas. Mine has never crashed, the tutorial is careful to explain things thoroughly. It's just that they neglected to include some important things.