Answer: understudy

AnswerCrossword Clue
UNDERSTUDYDramatic substitute.
UNDERSTUDYSometime substitute.
UNDERSTUDYActor waiting for a break
UNDERSTUDYBroadway backup
understudyto act another's part or take over another's duties
understudy(in the theater) A person who learns another's role in order to be able to act as a replacement at short notice