have a func_button that only the admin knows about, then give it OnDamaged | !activator | TargetName | Clive and set it to only once. You'll need a filter_activator_name, call it filter_clive and set the filter name to Clive. Then on the door's trigger set the activation filter to filter_clive. Now only the first person to shoot that button will be able to open the door. (Anyone will be able to walk through, but only they'll be able to open it)
You can make this harder for people to figure out by using triggers so players have to stand in the right place to shoot the button, or outputs to enable/disable the button so they have to do it quickly, or by adding more buttons that enable each other in sequence.