How can I set a value (drop down or single choice) to Null using a condition?

How can I set a value (drop down or single choice) to Null using a condition

Unfortunately, I am not sure if I understood the idea in full. As far as I know, you can change the dropdown's option on a condition, not sure as to setting it to null. Can you kindly clarify what you are trying to achieve, so we could search for possible options?