But what would be the equivalent of:

  (let ((quotient remainder (floor x y)))
    (+ (* quotient z)
       (* remainder (floor z y))))

Where floor is a function which returns multiple values rather than a list or vector of results?

Also, but this is entirely on me being an absolute dullard, I like having the brackets around each binding as a kind of guardrails for my mind to not lose track of where I am and what belongs where.

It's just:

    (let [[quotient remainder] (floor x y)]
      (+ (* quotient z)
         (* remainder (first (floor z y)))))

Because in Clojure there's no multiple value bind, when you want to return many results from a function, you wrap it in a vector, and that means you than just destructure that vector. That also means, it's not smart enough to know if it's used in a single value context or multi value context, so you need to always get the value out of the vector, which is why I call first in that second call to floor.