...A sissy on a singing ass will win
Veronica's napkin, pallidly drawn
cast-off of your unbound
feat, mis-spoken jockey (nailed
in the Campo every time).
The "sissy on a singing ass" fuses together : the loser-poet, St. Francis of Assisi, the "slow horse" (the donkey, Jesus's "colt" which carries him into Jerusalem as "King") of poetry (Pegasus), etc.
"Veronica's napkin, pallidly drawn" : the "palio" was the small cloth or prize or "pallium" given to the winner of the Palio race in Siena - the cloth linked with the Virgin (patron of city of Siena), & through her to "Veronica's napkin", the cloth which supposedly showed an image of Christ's face.
"cast-off of your unbound/feat, mis-spoken jockey" : generally thinking of the difference between the "real" Jesus and the various cultural/religious shapings, (mis)appropriations & distortions - throughout the long, tangled history of Jewish-Christian dialogue & dispute - of his "image" (much like current iconic celebrity-simulacra).
"nailed/ in the Campo every time" : the Palio race is, for the jockeys, a risky proposition in several ways - not least the risk getting beaten up by angry partisans of one horse or another. Also Jesus (the "nailed"/"feat" - the losing horse).
- this is, of course, just one sentence - there are other things going on here (echoes of Hart Crane etc.), & more (& different) things in the other stanzas....
p.s. you don't need to know all this stuff, carry all this baggage, to "get" the poem (or maybe you do). Often I don't "know" it myself very clearly. I relax into the poem, following certain parameters (like an algorithm). Like the bit about the "shadow faster than light". Came along when I happened to remember a NY Times op-ed piece from last week by Margaret Wertheim : shadows (because they are literally no-thing) actually can travel faster than the speed of light. The experiment (or thought-experiment?) she discussed had to do with a very powerful light beam circling on its axis in space. At a very great distance (on the perimeter of the light's "orbit") the shadow cast would travel faster - cover more distance - than the light itself.