ABC’s Nashville came back last night with a transitional mid-season premiere. But not before Lennon and Maisy, the singing siblings who play Maddie and Daphne, dropped a Charli XCX cover. The sisters’ subdued, stripped-down “Boom Clap,†uploaded earlier this week, has all the percussive flourishes you and Charli might expect. Lennon and Maisy’s lush harmonies, folk-y voices, and flawless chemistry, however, give this old earworm new life.