To play a supporting or minor role in relation to someone else: “Tired of playing second fiddle, she resigned and started her own company.” In an orchestra, the position of second violinist (fiddle) is not as glamorous as that of first violinist.
play second fiddle
Assume a subsidiary role to someone, as in Mary resented always playing second fiddle to her older sister. This term alludes to the part of second violin in an orchestra. Although many would argue it is as important as first violin, it is the idea of subordinacy that was transferred in the figurative term, so used since about 1800.