Is there a way to create a dropdown list box that displays instructional text which disappears on mouse-click?

Do you want the user to make a selection from a dropdown list box and based on the selection display some text then when the user clicks on an area in the text the text disappears? If so how about showing the text as an image in a button and when the user clicks on the button the button's visibility is set to hidden?

If you are interested in an example then you should check out the following PDF form that I created "Buttons with a Dropdown Field to Show Instructional Texts".

Yes, besides the tooltip you can use a format script to display text, but this is tricky.

First there needs to be a blank default entry.  A single space will do.

Next, add this to the custom format script

if(event.value == " ") event.value = "My Message";

I usually change the color as well so the help text is gray.

You can try using "tooltip' to add instructional information.

You will be able to find it in General tab for form field properties

For details refer, PDF form field properties, Adobe Acrobat