The definition is joy rides, referring to FERRIS WHEELS as fairground rides for enjoyment. For the wordplay, start with FERRARIS and remove AR (from "are mostly stolen" — ARE loses its last letter to give AR, and "stolen" tells you to remove it), leaving FERRIS. Then "Rolls" cleverly indicates WHEELS (rolls being a synonym, with the capital R acting as a deceptive device to make you think of the car brand). Put them together: FERRIS + WHEELS = FERRIS WHEELS.