>Does it make sense to have to learn Haskell and a Haskell framework just to understand Clojure?
No, but it makes sense to learn haskell to understand functional programming, which is what you asked. Clojure's emphasis on purity is essentially useless without a type system to go with it. The type system is what separates the pure functions from the impure ones.
Do you not understand what referential transparency is?
You can write "pure" functions in imperative languages. Any function which returns the same result for the same arguments is a pure function. You could write an entire pure functional program in C if you wanted.
No, but it makes sense to learn haskell to understand functional programming, which is what you asked. Clojure's emphasis on purity is essentially useless without a type system to go with it. The type system is what separates the pure functions from the impure ones.