Anthony Burgess does us a favor and tells us straight out what the title is about: it comes from the refrain of an old English folk song called "The Wanton Seed." If you've ever heard the old-timey, euphemistic aphorism that says young men have got to go out and "sow their wild oats," you'll have no trouble figuring out what the song is about.
For this cynical, dystopian story of a world where sexual reproduction is actively discouraged, Burgess replaces "wanton"with "wanting", using the latter word in its formal sense, to mean "missing" or "lacking." If humans curb their "natural" instinct to reproduce—so the novel's argument goes—we're in for some trouble.