Definition of Metaphor
Alex uses a metaphor to express his longing to hear classical music in Part 1, Chapter 3:
I wanted like a big feast of it before getting my passport stamped, my brothers, at sleep's frontier and the stripy shest lifted to let me through.
This sentence includes multiple metaphors. First, Alex describes wanting "a big feast" of classical music, reflective of his tendency to want not only to engage with but consume the things around him. Comparing music to a feast evokes its rich, sumptuous quality.
Second, he envisions getting his "passport stamped [...] at sleep's frontier." Because he often uses music to escape his gritty reality, it makes sense that Alex would compare the subsequent sleep to a kind of travel. As long as he is trapped with his rambling inner voice, only music can authorize him to cross the "stripy shest" or border, to leave that place and go elsewhere. It is likely that Alex and his droogs have never left the city and its surrounding areas, which allows unseen lands to take on more fantastical qualities in his mind.
Music often generates intense imagery in the novel, which Burgess hints at here by transitioning to more metaphorical language. At the same time, he provides another example of Alex's unique, Nadsat-inflected way of speaking.