The Oxford Thesaurus An A-Z Dictionary of Synonyms - understudyn.

1) second, substitute, stand-in, alternate, backup, double, sub, reserve, US pinch-hitter: The understudy got her break one night when the star fell ill. --v. 2) substitute for, stand in for, back up, double for, second, replace, US pinch-hit for: He is understudying Hamlet as well playing the part of Laertes.