From one drop down to another

enter the following event code into the VBA editor for sheet1 worksheet change

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$B$1" Then
Range("$B$2").ClearContents
End If
End Sub

hth
let me know how you go

(sorry I have no idea about the Pocket Excel - Hopefully one of the other's will have some idea)