WordNet (r) 3.0 (2006):

second string
    n 1: a squad of players that are available either individually
         or as a team to relieve or replace the players who started
         the game

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "second string":
   agent, alternate, alternative, analogy, back seat, backup, cast,
   change, changeling, common, company, comparison, complement, copy,
   counterfeit, crew, demeaning, deputy, disadvantaged, double, dummy,
   eight, eleven, equal, equivalent, ersatz, exchange, fake, fill-in,
   first string, first team, five, ghost, ghostwriter, humble,
   humbleness, humility, imitation, in the shade, inferior,
   inferiority, infra dig, junior, juniority, less, lesser,
   locum tenens, low, lower, lowliness, lowly, makeshift, metaphor,
   metonymy, minor, minority, modest, next best thing, nine, ordinary,
   personnel, phony, pinch hitter, platoon, proxy, relief,
   replacement, representative, reserves, ringer, rowing crew,
   second fiddle, second rank, second team, secondariness, secondary,
   servile, servility, sign, spares, squad, stand-in, string, sub,
   subaltern, subject, subjection, subordinacy, subordinate,
   subordination, subservience, subservient, substituent, substitute,
   substitution, succedaneum, superseder, supplanter, surrogate,
   symbol, synecdoche, team, third rank, third string, token,
   underprivileged, understudy, utility player, varsity, vicar,
   vice-president, vice-regent, vulgar