Alex, a teenage hooligan in a near-future Britain , gets jailed by the police. There he volunteers as guinea pig for a new aversion therapy proposed by the government to make room in prisons for political prisoners. "Cured" of his hooliganism and released, he is rejected by his friends and relatives. Eventually nearly dying, he becomes a major embarrassment for the government, who arrange to cure him of his cure. A pivotal moment is when he and his gang break into an author's home: the book he is writing (called "A Clockwork Orange") is a plea against the use of aversion therapy, on the grounds that it turns people into Clockwork Oranges (Orang is Malay for "Man"): they are not being good from choice (sentiments later echoed by the prison chaplain). The film reflects this: many bad scenes in a Clockwork Orange are accompanied by jolly music; if we are to experience them as we should, we have to do it consciously, by realising they are bad, and not because the director tells us so through the use of music and images.  

http://www.imdb.com/title/tt0066921/plotsummary

The disguise that Alex and his gang adopts when they attack people is of a gang of clowns. In the director's rendtition, he couples their costumes with jolly or upbeat music with their rampant acts of violence. One might assume that this juxtaposition may be to create a heightened sense of the madness of the situation, or perhaps to express these terrible events through the eyes of the deranged Alex, where these violent acts are actually pleasurable intrigues to him. The subversive figure of the clown therefore gives the audience an exaggerated understanding of the madness inherent in Alex's character. It's important to Kubrick to make is very clear that Alex is devoid of human compassion or any kind of moralistic code. The clown imagery becomes crucial to this in representing that they really see their exploits as a game or as fun, when to the audience they are completely abhorrent to every right minded person. This strong image of what Alex instinctively is contrasts the mechanical effect of Alex's behavioural psychology.