sing for one's supper
Work for one's pay or reward, as in Entertaining visiting scientists is part of the job; you know I have to sing for my supper. This metaphoric term alludes to wandering minstrels who performed in taverns and were paid with a meal. First recorded in 1609, it gained currency with the familiar nursery rhyme, "Little Tommy Tucker, sings for his supper" (c. 1744).