I'd suggest a clerk has no power to cancel a meeting in any circumstances once the notice of meeting has been issued but the clerk can or should advise that the meeting is believed to be incorrectly convened if that is the case. It should be noted, however, that there is case law (I can't remember the actual reference instantly) that states that a meeting is not necessarily illegal on the grounds of a mistake is made in the manner in which it is convened. Without further information on the circumstances, it's difficult to say whether the meeting can take place or not.