slot-makunbound

(slot-makunbound instance 'slot-name)

Makes the named slot unbound and returns the instance: slot-boundp then answers nil and a read through slot-value or an accessor signals unbound-slot. Storing into the slot binds it again.

On the JVM and WASM compilers the slot name must be a literal quoted symbol, like slot-value; a runtime-computed slot name works on the interpreter only.