Undergarments are called that for a reason- they are intended to be placed under a costume or under rehearsal attire, if necessary.
Too often, straps, briefs, shorts and more are seen in class or on stage, and it breaks the cleanliness of the intended look.
Even a nude leotard (or a clear strap) that shows can be distracting.
What is the easiest solution for this type of problem?
(1) If undergarments are required, tailor them to the look of clothing.
(2) Use pins, velcro, and tape to secure undergarments beneath the costume.
(3) Be self-aware and triple check everything! Do not assume something will stay in place. Rehearse, practice, and make adjustments to guarantee a successful appearance.