Get Custom Field Value

def eventIssue = Issues.getByKey(issue.key as String)
def expectedValue = 'some value you expect to see in the custom field'

//get custom field value
def value = eventIssue.getCustomFieldValue('customFieldName')

//clear custom field value
if(value != expectedValue) {
    eventIssue.update {
        eventIssue.clearCustomFieldValue('customFieldName')
    }
}