The ball valve is mounted inside a seperate tube which is the piece that you will be removing when you remove the retaining clip.
Even if using for a fermenter, you are probably going to want to open it up a bit. Most if not all sanke fermenters I've seen have a larger hole. Otherwise it will be next to impossible to clean, sanitize and inspect.
all kegs have a spring loaded ball valve caver95 is telling you correct. do a search on youtube for (keg to keggle) there is a guy named bobbyfromnj that will show you how to pop that ring off and get the stem out. should make a great fermenter for you