The only solution not involving code is:

Create the column as:

 - Choice
 - Not required
 - Unique
 - Choices: 1, 2, 3, 4 and 5 (not None)
 - Drop-Down Menu
 - Clear default value

![enter image description here][1]

That'll allow the users to create any number of entries without a priority, but only one for each of the 5 values.

This seems is like you want it to work. **BUT** once an entry has been assigned a priority, there is no way to clear it again.
 [1]: https://i.sstatic.net/izqts.png