What should I look for in an Online Erectile Dysfunction clinic?

It should be possible to locate the website provider (check for a physical address, a contact number, a registration number if it is a pharmacy). Dubious merchants will often operate from abroad to escape legal pursuit.

The prescription should be written by a doctor who is registered in the country the treatment is being dispatched from
If the site is UK compliant:

The payments should be processed via a secure server.
Never give out any passwords or credit card details by email or live chat as this is not secure hence the information could be used for fraudulent purposes

The medicine should originate from a legal source.
The only real proof of parallel imports is sometimes displayed on the boxes. The instructions may be written in a different language or they may have a different country for sale indicated. Finally, all Cialis and Viagra sold in Europe is sold in divisions of four. Most of the rest of the world sells their treatments in divisions of five (5, 10, 15, 20).

A legitimate online pharmacy should insist that prior to placing an order online or viewing treatment prices, you answer questions about your medical history.
They will send your information to a licensed doctor for review and you will only be given a prescription for that medication if the doctor approves your order. An easy way to avoid the illegal and irresponsible operators in the online pharmacy business is to follow this rule: don't use online pharmacies which do not offer access to a registered pharmacist or doctor to answer your questions.