Why does the EU need a clearing centre to circumvent US sanctions against Iran? As reported here and elsewhere, the EU is considering the creation of a clearing house to keep trading with Iran despite US sanctions. Why is it not possible for the EU to conduct normal financial transactions with Iran without the involvement of US institutions?
Because the US has threatened to sanction European companies that trade with Iran.  Most European companies that are interested in trading in Iran also trade with the US, so would be vulnerable to various kinds of sanctions.