Unable to clear combobox content with Backspace

I would like to be able to claer my combobox content by clicking on my backspace key. Right now I am stuck with a partially blue shaded word. please help

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Moderator's Edit: I have moved your thread to the right subforum. Also, use code tags when posting code. To do so in future, select your code and click on the # icon at the top of your post window.