If you're looking for working examples of how to reply to cljs-ajax from a ring server, take a look at the integration test server source code.
If you're using transit, take a look at ring-transit. It will populate the request :params with the contents of the transit request.
For handling JSON requests use ring-json middleware instead. This populates the data in the :body tag. However, note that it does not provide protection against JSON hijacking yet, so do not use it with JSON format GETs, even for internal websites. (As an aside, if you need lower level JSON access, e.g. for formatting, we'd recommend Cheshire over data.json.)
For using EDN then you may wish to take a look at ring-edn middleware. It will populate the request :params with the contents of the EDN request.
If your tastes/requirements run more to a standardized multi-format REST server, you might want to investigate ring-middleware-format.
By default the browser blocks ajax requests from a server which is different to the current page. To enable such cross origin requests add the Access-Control-Allow-Origin and Access-Control-Allow-Headers headers to your response as follows. You can use the ring-cors library to wrap your routes with customizable CORS middleware.
(require '[ring.middleware.cors :refer [wrap-cors]])
(def allowed-origins [#"https://example-a.com" #"https://example-b.com"])
(def allowed-methods [:get :post :put :delete])
(def allowed-headers #{:accept :content-type})
;; my-routes already defined somewhere
(def handler
(wrap-cors my-routes :access-control-allow-origin allowed-origins
:access-control-allow-methods allowed-methods
:access-control-allow-headers allowed-headers)
Access-Control-Allow-Origin is the standard header telling the browser to permit a cross origin request. For Google Chrome we must include the header Access-Control-Allow-Headers to prevent it stripping the Content-Type header from our requests. For non-simple cross-origin requests (e.g. GET or HEAD requests) the browser will submit two requests: a preflight and the target request. The first is an OPTIONS request submitted in order to probe the endpoint. The second is the main GET or POST request. To understand this process better, you can read Mozilla's guide on Cross-Origin Resource Sharing (CORS).