1. [n]  an actor able to replace a regular performer when required
2. [v]  be an understudy or alternate for a role

UNDERSTUDY is a 10 letter word that starts with U.

1. \Un"der*stud`y\, v. t. & i. (Theater)
   To study, as another actor's part, in order to be his
   substitute in an emergency; to study another actor's part.
   
   

2. \Un"der*stud`y\, n.
   One who studies another's part with a view to assuming it in
   an emergency.