From the A.Word.A.Day mailing list
second fiddle (SEK-und FID-l) noun
Secondary role. A person in such a role.
"Until last year the club had played second fiddle to their great rivals Leicester in recent times."
Ian Malin, "Rugby Union," The Guardian (UK), Mar 10, 2001.
Presumably this derives from orchestra practice of having a first chair, second chair, etc. for instruments, with the first chair given to the most skilled player and so forth in decreasing virtuosity.