I also could not get RadComboBox.ClearSelection() to work, Did all the steps mentioned in this thread.
Got it to work with the following code. Posting is here as it might help others
One important point that may be relevant is that I am using Checkboxes in my Combobox.
Protected Sub ClearRadCB()
Dim RadCB As RadComboBox = RadComboBoxRelations
If Not RadCB Is Nothing Then
For i = 0 To RadCB.Items.Count - 1
If RadCB.Items(i).Checked Then
RadCB.Items(i).Checked = False
Then you can call ClearRadCB() in your code or paste in the event procedure.