public Task ClearAsync()

ArcGIS.Desktop.Editing.Attributes.Attribute attribute = null;
var fi = new Inspector(false);
try
{
  await fi.LoadSchemaAsync(lyr);
  attribute = fi.Attributes.c(a => a.FieldName == "My fieldName");
  await fi.ClearAsync();
}
finally
{
  fi = null;
}