Dependent data validation question

You'll need to use Visual Basic for that. Here's one solution.
Right click on the sheet tab and choose "View Code"
Paste this code

Private Sub Worksheet_Change(ByVal Target As Range)
If Target = Range("A2") Then
Range("B2").ClearContents
End If

End Sub

This assumes your initial list is in A2 with your dependent list in B2. Does this work for you?