How to make reference/pointer in clojure?

How to make reference/pointer in clojure?

I would like to change normal behavior of code
(def a 5)
(def b a)
(def a 1)
b
5
To this behavior
(def a 5)
(*something* b a)
(def a 1)
b
1
It is just for learning purposes so please do not try any deep sense in this.