understudy
n.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.”
“Nottingham Forest could be about to sell Morato , their understudy centre-back, to Roma.”
“The 23-year-old England goalkeeper, currently understudy to Jordan Pickford in Tuchel’s squad, shone at Burnley in the Championship and arguably deserved more than to be No 2 at City last term.”
From under- + study.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.