An additional set of brackets allows you to define a function with a variable number of arguments. The following example defines a function that can take one argument or two arguments:
(defn foo ([x] x) ([xy] (+ xy)))
You can see this as a definition of two functions under the same name. The corresponding function will be called depending on the number of arguments that you provide.
If you define a function with a fixed number of arguments, the following two forms are equivalent:
(defn bar ([x] x))
and
(defn baz [x] x)
With that in mind, you can understand the compiler exception. You are trying to define a function as follows:
(defn qux (([x] x)))
When using an additional set of parentheses, the closure expects the first element inside the partensis to be a vector (in brackets). However, in this case, the first element is ([x] x) , which is a list, not a vector. This is the error you are getting.