I'm pretty sure deps-new does that: https://github.com/seancorfield/deps-new

a common way is two separate arities, and the n-1 arity calls the n arity with the default value

ah ok!

as a contrived example:

(defn frob
  ([x] (frob x 5))
  ([x y] (vector x y)))
(frob 1 2)
=> [1 2]
(frob 1)
=> [1 5]

I tried with associative destructuring and couldnt get it to work.

since the :or part of associative destructuring is in the map argument, it still requires the argument be passed - but with the n-1 arity, that can pass the empty map, which will cause the :or part of the destructuring to be used

(defn frob [x & [y]] ...) will give you y bound to nil if it isn't passed in which might be able to stand in for an empty map, depending on what you want to do to it.

(but I generally prefer the multi-arity approach Bob suggested since it makes the default values clearer)