WordNet (r) 3.0 (2006):

second fiddle
    n 1: someone who serves in a subordinate capacity or plays a
         secondary role [syn: second fiddle, second banana]
    2: a secondary role or function; "he hated to play second fiddle
       to anyone"

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "second fiddle":
   back seat, cog, commonality, commonalty, creature, flunky,
   follower, hoi polloi, humbleness, humility, inferior, inferiority,
   junior, juniority, lightweight, lower class, lower orders,
   lowliness, masses, minority, pawn, second string, secondariness,
   secondary, servility, subaltern, subjection, subordinacy,
   subordinate, subordination, subservience, third string,
   third stringer, underling, understrapper, yes-man