This is a tutorial shows how to clear a custom field

Assumption

The custom field in this tutorial is called Database Version of type Single Select list on this instance ID: 10301

Steps

Create a new Action by clicking CREATE button
Enter a name: Clear field and click CREATE
Select Method: PUT and URL: <your_base_url>/rest/api/2/issue/{{issue.key}}
From the Atlassian REST API we can read that to set a custom field we need to call PUT /rest/api/2/issue/{issueIdOrKey}
Select Authentication method: Basic Auth, specify an admin user that can add users to groups in JIRA.
Add request headers, key: Content-Type with value application/json

On the Body section add the following

{
    "fields":{"customfield_10301":""}
}

Click Triggers tab
Select ISSUE RESOLVED event, in this example we will use the Issue Resolved to clear the field.
On JQL Condition. When to execute the event(s) write Project = TESTB and to limit the trigger to the TESTB project
Click SAVE
(Optional) Click Test button, Issue Key needs to be provided and click Send, Then go Log Tab.