Giovanni Gherardini has written:

'La gazza ladra = The thieving magpie'

La Gazza Ladra Overture - 1954 is rated/received certificates of:

USA:Approved (PCA #17090)

The cast of La Gazza Ladra Overture - 1954 includes: Johnny Green as Himself - conductor The MGM Symphony Orchestra as Themselves

It's from Rossini's opera "La Gazza Ladra" ("The Thieving Magpie"). The melody can be heard in the overture. I don't know where it appears in the opera itself.

Pretty sure it is the Overture to La Gazza Ladra (The Thieving Magpie) by Gioacchino Rossini

It's a reference to the 1971 film "A Clockwork Orange" and that was the song that was used during that scene where Alex puts his Droogs in place.