Be honest in the interview and admit why you had an unsat. Busts can and will happen in a career. More than one and you will have a hard time explaining that "it was just a bad day". Reference Colgan and Pinnacles crashes. Both commanded by Captains who had "five bad days" and their sub par performance led them to the final bad day. An unsat should be a time of introspection. Ask yourself why you failed and how you can improve your performance. Don't let it happen again. Some guys have bad days and others are genuinely unprepared. Either way, improve the performance.