second fiddle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A secondary role.
• noun One who plays a secondary role.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music A fiddle part in harmony to the first fiddle.
• noun music The person playing second fiddle.
• noun idiomatic A sidekick or subordinate, or the role of such a person.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a secondary role or function
• noun someone who serves in a subordinate capacity or plays a secondary role