register.el

;; ‘C-x r s <register-key>’ save to register
;; 'C-c C-j x <register-key' to send to repl
(defun cider-insert-register-contents (register)
  (interactive (list (register-read-with-preview "From register")))
  (let ((form (get-register register)))
    ;; could put form into a buffer and check if its parens are
    ;; balanced
    (if form
        (cider-insert-in-repl form (not cider-invert-insert-eval-p))
      (user-error "No saved form in register"))))

(define-key 'cider-insert-commands-map (kbd "x") #'cider-insert-register-contents)
(define-key 'cider-insert-commands-map (kbd "C-x") #'cider-insert-register-contents)
(define-key cider-repl-mode-map (kbd "C-c C-j") 'cider-insert-commands-map)