As a poet using your art as an expression of your thoughts and feelings, you may not always be understood. Because of this, poets constantly risk absurdity, or in other words, run the risk of sounding absolutely nonsensical.

Lawrence Ferlinghetti uses this title and opening line to demonstrate the risk he, along with all poets, takes in producing such literature, as it may become misconcieved.

Ferlinghetti seems to create a circus of some sort, but rather than a typical circus with clowns and elephants, he creates a circus in his mind. The circus of the poet's mind has a much more creative and breathtaking potential, although, it may be tougher to convey to the public who may not understand his thoughts.
The magical acts and "death-defying" works that take place are the poet's thoughts, and unlike a normal circus, there is no assurance of safety - when he displays his thoughts, there is no telling how they will be recieved.
Ferlinghetti states that this "circus" is "where Beauty stands and waits with gravity," possibly referring to a trapeze artist, metaphorically representing his illustrious thoughts.

The poem opens with a focus on the poet who's compared to an acrobat of sorts. He's got some fancy footwork, which we know is just a metaphor for his magic-hat of poetic devices. He also has some lofty responsibilities, catching Beauty, and is constantly risking "absurdity and death." We know from the very beginning that Ferlinghetti is trying to make us see his poet in a more imaginative and figurative sense. Which makes sense since the poet-acrobat's entire purpose is by nature a little crazy and unpredictable.

As he "performs above the heads of his audience," this high wire is everything the poet-acrobat rests upon in terms of his work and purpose. At the same time, it's high so it's full of danger and potential mishaps. That wire is everything to the poet, because it will take him to Beauty, who's waiting at the other end. But it also is the very thing that keeps the poet in the mindset of "constantly risking absurdity."
There's nothing easy about his performance. It's also of "his own making" so there's no blaming anyone else if the poet/acrobat happens to fall to his figurative death.