understudy
nounUK /ˈʌndəstʌdi/
US /ˈʌndərstʌdi/
An understudy is a person who learns an actor's part so they can act in their place if needed.
The understudy went on stage when the lead actor got sick.
OTHER MEANINGS
2. To learn a role in order to replace another performer if necessary.
She understudied the main character for the whole season.
3. A person who is trained to take over another person's job or duties.
He served as an understudy to the manager for a year.
4. To act as a substitute for someone in a performance or task.
The young dancer understudied the principal ballerina.