Webster's English Dictionary

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understudy

1. un.der.study \-.st*d-e-, .*n-d*r-'\ vi : to study another actor's part 
   in order to be his substitute in an emerg ency to prepare (as a part) as 
   understudy; also : to prepare as understudy to (as an actor)
2. un.der.study \'*n-d*r-.st*d-e-\ n : one who stands prepared to act 
   another's part or take over another's dut ies