Clean is written in the key of B Minor.
According to the Theorytab database, it is the 5th most popular key among Minor keys and the 13rd most popular among all keys. Minor keys, along with major keys, are a common choice for popular music. The three most important chords, built off the 1st, 4th and 5th scale degrees are all minor chords (B minor, E minor, and F♯ minor).
See the B Minor Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!