There is a difference between an extraordinary meeting, as defined by legislation, and an additional ordinary meeting. The Clerk absolutely does have the authority to convene an additional ordinary meeting. This is a meeting which is within the usual remit of the council but is sometimes necessary where ordinary business has to be dealt with urgently. So, for example, many councils don't meet in July or August but if there was an urgent planning application that requires a response or perhaps a financial matter that needs to be dealt with, both are "ordinary" business of the parish council but the clerk may have to convene an additional meeting to deal with this, particularly if there is no scheme of delegation in place enabling the Clerk to respond within the timeframe.