Complex sentence: Charles, whose royal assent was required for the bills to become law, was only willing to consider such an assembly if the members were chosen by the clergy, as was done in Convocation, the assembly of clergy of the Church of England.
Split into simpler sentences: Charles, whose royal assent was required for the bills to become law, was only willing to consider such an assembly if the members were chosen by the clergy. This was the practice for selection of members of Convocation, the assembly of clergy of the Church of England.