Is the Reusable Element a Repeating Group?
Is the Repeating Group on the Page and you have placed a reusable element into the repeating group?
If you have the repeating group inside of a reusable element and you put that reusable element onto the page and you want the user to select a value from the repeating group which is inside of the reusable element and you want that value for some event that is on the page, then best approach is to put onto the reusable element a custom state that is the same type of data as that of the repeating group you created inside of the reusable element.
When user selects the value from the repeating group set the custom state of the reusable element itself to what was selected. Then on the page you can reference the custom state value of the reusable element.