verb (used with object), understudied, understudying.
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.
verb (used without object), understudied, understudying.
to act or work as an understudy.
noun, plural understudies.
a performer who learns the role of another in order to serve as a replacement if necessary.

1870–75; under- + study

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World English Dictionary
understudy (ˈʌndəˌstʌdɪ)
vb , -studies, -studying, -studied
1.  (tr) to study (a role or part) so as to be able to replace the usual actor or actress if necessary
2.  to act as understudy to (an actor or actress)
n , -studies, -studying, -studied, -studies
3.  an actor or actress who studies a part so as to be able to replace the usual actor or actress if necessary
4.  anyone who is trained to take the place of another in case of need

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

1874, in the theatrical sense, from under + study (v.). The noun is attested from 1882.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
In photography, chance is often the overlooked understudy of talent.
But it is usually a bad sign for your candidacy if you're only sticking around
  as an understudy.
We will review some of the more promising techniques understudy.
It is therefore important to test questionnaires and, if necessary, adapt them
  to each population understudy.
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