un·der·stud·y 
(ŭndər-stŭd′ē)
(ŭndər-stŭd′ē)v. un·der·stud·ied, un·der·stud·y·ing, un·der·stud·ies
v.tr.
1. To study or know (a role) so as to be able to replace the regular performer when required.
2. To act as an understudy to.
v.intr.
To be engaged in studying a role so as to be able to replace the regular performer when required.
n. pl. un·der·stud·ies
1. A performer who understudies.
2. A person trained to do the work of another.