apart from the answers provided by others, I think you are doing extra work to get ITEM_CODE, you can simply change the sql statement as
string query = "SELECT DESCRIPTIO, ITEM_CODE FROM stock ORDER by DESCRIPTIO ASC";
then set both DisplayMember and ValueMember
comboBox_Supp1.DisplayMember = "stock.descriptio";
comboBox_Supp1.ValueMember = "stock.item_code";
then you can set get the combobox selected item value(ITEM_CODE) or value(ITEM_CODE) of any item by index
textBox1.Text = comboBox_Supp1.Items[0].Value.ToString();
or
textBox1.Text = comboBox_Supp1.SelectedValue.ToString();