This I am afraid is a grey area. The GDPR legislation isn't as black and white as many had hoped. Until that happens there will, I guess by some understanding for genuine and low-level errors.
In response to your specific question - The Clerk should have asked themselves the following question: Do the Councillors NEED that information? (In this case, email address - in other cases could be the name and address). The short and simple answer is no. The Clerk, being the employee of the council and the initial recipient of the email is the only person who needed that information. The Clerk should have simply copied and pasted the content of the email redacting any information that could be considered personal and therefore unnecessary.
That said if you were to report the breach to the Information Commissioners Office they would ask two initial questions...
1. Have you contacted the person who submitted the email to explain it was passed to Councillors?
2. By sharing the email address in the way that you did has it caused any damage to the person who emailed i.e. increased unwarranted marketing calls, harassment or a danger to life for example?
The answer to number two is probably little to none for the example you have described and therefore the ICO would be unlikely to investigate. It would, however, log the incident to ensure this is not repeated.
Actions you need to take are:
1. Inform the Clerk and the Council that it is a breach.
2. Request that the Clerk email the person that sent the email explaining that they had sent it to all Councillors by mistake and that it will not be repeated.
3. Request that the Clerk creates a GDRP policy (Urgently) and have it signed off and implemented by the council.
4. If the above does not happen then explain to the council that you will report the incident to the ICO.
I reiterate this is a low-level breach which can be remedied very easily and very quickly but it is important to understand that it is still a breach.
Hope this helps.