user=> (defn make-list [] '(1 2 3))
#'user/make-list
user=> (defn get-list [] (map #(str "foo" % ) make-list))
#'user/get-list
user=> (get-list)
IllegalArgumentException Don't know how to create ISeq from: user$make-list clojure.lang.RT.seqFrom (RT.java:505)
user=> (defn get-list [] (let [ml (make-list)] (map #(str "foo" % ) ml) ))
#'user/get-list
user=> (get-list)
("foo1" "foo2" "foo3")
Why can the value from defn not be accessed in the same way from the BODY of a function as it can from the LET clause?
defn declares a function so you need to invoke it in order to get its result:
(defn make-list [] '(1 2 3))
(defn get-list [] (map #(str "foo" % ) (make-list)))
(get-list)
;; ("foo1" "foo2" "foo3")
But since your makeList function doesn't do anything but return a list, you can use def instead as it simply declares a var:
(def make-list '(1 2 3))
(defn get-list [] (map #(str "foo" % ) make-list))
(get-list)
;;("foo1" "foo2" "foo3")
Lastly, use kebab-case instead of camelCase for function and var names in Clojure. That's the convention unless you're declaring Protocols or Types.