to learn (a role) in order to replace the regular actor or actress when necessary.

to act as understudy to (an actor or actress)

to understudy the lead.

to act or work as an understudy.

a performer who learns the role of another in order to serve as a replacement if necessary.

noun

alternate , backup , double , fill-in , pinch hitter , replacement , reserve , stand-by , stand-in , sub * , successor , substitute