'In Worksheet Module
'-------------------
Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Address = "$A$1" Then
        If scheduled > 0 Then Application.OnTime scheduled, "ClearIt", Schedule:=False
        scheduled = Now + TimeSerial(0, 0, 5)
        Application.OnTime scheduled, "ClearIt", Schedule:=True
    End If
End Sub

'In Standard Module
'------------------
Public scheduled

Sub ClearIt()
    Application.EnableEvents = False
        Sheet1.Range("A1").Value = ""
    Application.EnableEvents = True
    scheduled = 0
End Sub