MahApps/MahApps.Metro

@TripleNico You can get this by using this converter (works only with non editable ComboBox)

[ValueConversion(typeof(object), typeof(bool))]
public sealed class IsNotNullConverter : IValueConverter
{
    /// <summary>
    /// Gets a static default instance of <see cref="IsNotNullConverter "/>.
    /// </summary>
    public static readonly IsNotNullConverter Instance = new();

    public object? Convert(object? value, Type targetType, object? parameter, System.Globalization.CultureInfo culture)
    {
        return value is not null;
    }

    public object? ConvertBack(object? value, Type targetType, object? parameter, System.Globalization.CultureInfo culture)
    {
        return Binding.DoNothing;
    }
}

usage:

<ComboBox Width="200"
          Margin="{StaticResource ControlMargin}"
          mah:TextBoxHelper.ClearTextButton="{Binding Path=SelectedItem, RelativeSource={RelativeSource Mode=Self}, Converter={x:Static local:IsNotNullConverter.Instance}}"
          mah:TextBoxHelper.Watermark="Please select an item..."
          SelectedIndex="0">
    <ComboBoxItem Content="Item 1" />
    <ComboBoxItem Content="Item 2" />
    <ComboBoxItem Content="Very long Item 3 for MahApps.Metro" />
    <ComboBoxItem Content="Item 4" />
</ComboBox>