Having said that, Business Rules also came with some limitations, one such limitation is the inability to clear field value. Lots of feedback’s were provided and finally the developers have listened and they have introduced this functionality in the latest update i.e. CRM 2015 Update 1.
Clear Field Value
Clearing a field is nothing but setting it’s value to null. So, all you need to do is to select Set Field Value and then previously it used to display only two options for any given field i.e. Field and Value but now you should be able to find a new option that reads Clear. Take a look at the screenshot below.