Use mount to control an application's "mode", and an atom to control feature knobs for each mode.

config.clj

(ns server.config
  (:require [mount.core :refer [defstate]))

(defstate mode :start :prod)
             
(def switchboard (atom {:dev {:feature-one :setting-a}
                        :prod {:feature-one :setting-b}}))

(defn set-feature! [name value]
  (assert (not (instance? mount.core.DerefableState mode)))
  (swap! switchboard assoc-in [mode name] value))
            
(defn feature [name]
  (assert (not (instance? mount.core.DerefableState mode)))
  (get-in @switchboard [mode name]))
  
(defn feature-dispatch [name]
  (fn [& _] (feature name)))

core.clj

(ns server.core
  (:require [server.config :refer [feature-dispatch]]))

(defmulti feature-one (feature-dispatch :feature-one))
(defmethod feature-one :setting-a [args] ...)
(defmethod feature-one :setting-b [args] ...)
(defmethod feature-one :default [args] ...)

user.clj

(ns user
  (:require [mount.core :as mount :refer [swap]]
            [server.config]))

(defn start! []
  (-> (swap {#'server.config/mode :dev})
      mount/start))

(defn stop! []
  (mount/stop))

(defn mode [m]
  (mount/start-with {#'server.config/mode m}))

(def set-feature! server.config/set-feature!)