Complex Binding

I have textbox bind to a listbox with expander controls. I'm able to get the Textbox populated with the string using the following:

Text="{Binding ElementName=RecBOX, Path=SelectedValue, Mode=Oneway}"

Instead of SelectedItem I use Selected Value.
The Listbox is binded with xml file not much code behind it.
The problem is, the textbox populates the whole string. How can I choose what part of the string to populate in he textbox
 
Any thoughts?