The Collaborative International Dictionary of English v.0.48:

Understudy \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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Understudy \Un"der*stud`y\, n.
   One who studies another's part with a view to assuming it in
   an emergency.
   [1913 Webster]

WordNet (r) 3.0 (2006):

understudy
    n 1: an actor able to replace a regular performer when required
         [syn: understudy, standby]
    v 1: be an understudy or alternate for a role [syn:
         understudy, alternate]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "understudy":
   act for, advocate, agent, alter ego, alternate, alternative,
   amicus curiae, analogy, answer for, appear for, attorney, back up,
   backup, backup man, bit player, champion, change,
   change places with, changeling, commission, comparison, copy,
   counterfeit, crowd out, cut out, deputize, deputy, displace,
   double, double for, dummy, equal, equivalent, ersatz, exchange,
   executive officer, exponent, extra, fake, figurant, figurante,
   figurehead, fill in for, fill-in, front for, ghost, ghostwrite,
   ghostwriter, imitation, lieutenant, locum, locum tenens, makeshift,
   metaphor, metonymy, mute, next best thing, paranymph, personnel,
   phony, pinch hitter, pinch-hit, pinch-hit for, pleader, procurator,
   proxy, relief, relieve, replace, replacement, represent,
   representative, reserve, reserves, ringer, second,
   second in command, second string, secondary, sign, spares,
   speak for, spear-carrier, spell, spell off, stand in for, stand-in,
   standby, sub, subrogate, substituent, substitute, substitute for,
   substitution, succedaneum, succeed, supe, super, supernumerary,
   supersede, superseder, supplant, supplanter, support,
   supporting actor, supporting cast, surrogate, swap places with,
   symbol, synecdoche, third string, token, understudy for,
   utility man, utility player, vicar, vicar general, vice,
   vice-president, vice-regent, vicegerent, walk-on,
   walking gentleman