This wiki will provide the details about how to clear the selection field values when you click an action called Reset.
Typically if you are familiar with HTML coding, you specify Reset in the input type. Similarly we can get the same functionality in Webdynpro ABAP for clearing the input field values on the selection screens.
Please follow the blog, https://www.sdn.sap.com/irj/scn/weblogs?blog=/pub/wlg/2868 for how to create a select-options in Webdynpro ABAP.
In that tutorial, create a button called Reset and attach Action called Reset.
On the action reset put in the following code.