What is Understudy ?
Understudy is noun an actor who learns a part in the play so as to be able to act it if the usual actor cannot perform The understudy had to take over last night when the male lead fell and broke his arm. Synonym substitute verb to be the under- study for an actor Who is understudying Judi Dench? He is understudying Hamlet.
source: Easier English, Student Dictionary Upper Intermediate Level