You can apply through clearing after 30 June 2018 providing you are holding no offers or did not meet the entry requirements.
Unlike normal entry you will not apply through UCAS, you will need to be placed in Clearing, but you will call universities to see if they were able to offer you a place. If they are, they will make you an offer through UCAS which you need to accept. Like Extra, there is no limit to how many applications you can make through Clearing, however, you can only hold one offer.
Similar to Extra, you should not use Clearing as your 6th choice. There is no guarantee the courses you want will be released into Clearing. Your applications through the year should be the 5 places you want, Extra and Clearing are subject to how many places universities may have past 15 January 2019. If they fill up fast, they will not be open.
If it looks like Clearing will be needed, again, do not panic. Many other applicants will be in the same situation as you. Try to remain calm, and plan your next steps. Try and keep the days around results days free so you are able to secure an application with time to spare.