Tim Bowen sheds some light on the origins and definition of the phrase to play second fiddle.
“For most of the match United were forced to play second fiddle to a resurgent Arsenal team”. Here the implication is that Arsenal dominated the match and United’s role in the proceedings was a subordinate one.
The expression to play second fiddle can be used in a wide variety of contexts and means to be less important or less powerful than someone else. Unsurprisingly, the expression is generally held to have its origins in the world of music. In early orchestras the most prominent and high profile member of the orchestra was the musician who played the lead (or first) violin. Naturally, there would also be other violinists in the string section of the orchestra but these would not be as important or honoured as the lead violinist. These other violinists became known as second fiddles and the expression has now become firmly established in the language.