Q: 如何清除 DBLookupComboBox 內容?
A: 你可以在 OnKeyDown 事件中處理, 假設要讓 user 按 Delete 鍵時清除:

If you're connected to a dataset then clear the datafield in the db:

procedure TForm1.DBLookupComboBox1KeyDown(Sender: TObject; var Key: Word;
  Shift: TShiftState);
begin
  if Key = VK_DELETE then 
  begin
    with Sender as TDBLookupComboBox do 
    begin
      if not(DataSource.DataSet.State in dsEditModes) then
        DataSource.DataSet.Edit;
      DataSource.DataSet.FieldByName(DataField).Clear;
    end;
  end;
end;