I've actually figured out my the problem now. As usual, as soon as I have up and asked about it I spotted it. But still would be useful/ interesting to know for future. 🙂

when a repl fails startup, that is almost always because your tools told the repl to pre-load a namespace that was broken

if you still have a repl prompt after the error you can use (require 'my.ns :reload-all) to get the error again, and *e will hold the detailed error object

you can then edit your code, and repeatedly require / reload the apropriate namespaces until everything loads correctly, without restarting the repl itself

as a last resort (eg. if you can't even get a repl prompt) you can start clojure.main , the -main in that ns gives you a very bare bones clojure repl that you can do your require / reload in until things are fixed (you might not even be able to use arrow keys to edit though...)