The attached screenshot shows you where the score sets menu (and access to the score sets editor) is located. The menu normally says "All Instruments". That'll look different when we're done...
Suggest you do the following in a separate score editor window instead of the embedded score editor in the arrange page:
1. Right now the score sets menu will say "all instruments". Click on any measure of the part you want to see. Invoke the following function in the score editor's local menu: [b]Layout > Create Score Set from Selection[/b]
2. Now look at the score sets editor and it will say "___name of your instrument___"
3. Double-click on the background of the score editor window, or, click the hierarchy arrow. Now you'll see only the regions for that part.
VOILA! Well, almost...
4. Double-click on the score sets menu where it says the name of your instrument. This will open the score sets editor. Once in there you rename the part's 'long name' and 'short name'.
5. Suggest you rename the score set itself if it doesn't display a descriptive-enough name, i.e., "alto section" or whatever.
6. [b]The most important part[/b]: from this point forward, when you want to view just that part, make sure that "alto section" is selected in the score set menu.