<my:AutoCompleteBox Grid.Row="0"
                                Grid.Column="1"
                                HorizontalAlignment="Left"
                                Margin="0,6,0,0"
                                Name="acTown"
                                SelectedItem="{Binding NewTown, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                                ValueMemberBinding="{Binding Converter={StaticResource TownConverter}}"
                                Populating="Populating"
                                VerticalAlignment="Top"
                                Height="Auto"
            </my:AutoCompleteBox>

public void ClearTown()
    {

    NewTown = null;

    OnPropertyChanged("NewTown");
}