understudy
n. countablen. a person who learns to do a job or a role so they can take over if the main person is sick or busy. In a theater, this is the person who practices the lead role to be ready for a show.
n. a person who is trained to perform the duties of another, typically in a professional or theatrical context. Often used to describe a performer who is prepared to replace the lead actor in a production.
She is the understudy for the lead role in the play.
When the lead singer fell ill, the understudy stepped in and performed the entire set without missing a note.
While the lead actor received the majority of the press attention, the understudy spent every evening rehearsing the entire script to ensure a seamless transition should an emergency arise.
From under- + study.