Hi -
Add an entry to your drop down list where both the value and display are blank. When first create a drop down list control, the only value is Select ... with a blank value. If you clear "Select ..." out of the display on that entry, you have a blank choice. Set it to the default.
You'll have to add rules to the field bound to the drop down that when it's value is blank, you set the values of the other fields to blank.