If you are def
-ing global vars inside your function, that's generally considered bad practice and reason enough to use let
as you suggest instead.
However, you can capture a snapshot the mappings of your namespace.
(def ns-snapshot (ns-map *ns*))
So that after you intern symbols
(def foo 1)
(def bar 2)
You can determine the additions
(reduce dissoc (ns-map *ns*) (keys ns-snapshot))
;=> {bar #'so.core/bar, foo #'so.core/foo}
And un-map them
(doseq [[k v] (reduce dissoc (ns-map *ns*) (keys ns-snapshot))] (ns-unmap *ns* k))
So that you'll get the desired undefined error again
foo ;=> CompilerException ... Unable to resolve symbol: foo in this context