You are eligible for Clearing if you have applied in the current application cycle and if you have paid the full application fee (£23 for 2013) and you have not withdrawn your application, and at least one of the following:
If you originally applied for one choice, paid £12 (for 2013), and you are not holding an offer, you need to make the additional payment of £11 which can be done in Track using a debit or credit card.
Track will tell you if you are eligible - if you are, an 'Add Clearing choice' button will appear on your 'choices' screen which you can use to apply for a course. Your Clearing Number will also be displayed.