day job

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One's primary job, usually at daytime and enabling one to pursue a secondary activity from which one would eventually like to draw an income: "She took a day job as a bank teller in Lubbock; nights, she played in pickup games, but she didn't have a trainer, a coach, or a team” ( Elizabeth Royte).

Wiktionary

1. n. One's main mean of income, during the daytime.