Have you tried using a Pop up menu with the first column of your two column value list hidden? That isn't a good choice in every case, but it could do what you describe if I've read your post correctly.
Yes, that does work in my applicaiton thank you. It does raise a new question however,
With a drop-down list, I can click into the field and hit delete to clear the value stored into it. This does not seem to be possible with the pop-up menu. I've not tried this on my Mac yet, but most of my users are on Windows machines so I thought I would start there first.
You can use the delete key to delete the contents of a pop-up menu. Simply select a value (any value) from the pop up to give it the focus and then press the delete or backspace key immediately afterwards.
This trick also works to clear radio button value selections.