Some people take a very rigid view of this, but my view is that the legislation does not require it. It is quite correct that a council cannot lawfully make a decision unless the matter has been clearly identified in an agenda, issued with the required notice of at least three clear days.
To encourage compliance with the law, councils have been urged to avoid having an agenda item of "Any Other Business". That is not a sufficient description of what might be decided to be a basis for any lawful decision.
However, it is important for councils to be able to function effectively. The council of which I am a member has two standard items that are somewhat general.
After the minutes have been agreed, there is "Matters Arising". This does not permit decisions, but is important to highlight what progress has been made and what matters are in danger of being forgotten.
Towards the end of the meeting is "Questions to the Chairman". Again, decisions cannot be taken, but new matters can be raised, and if the council wishes, discussed. A possible outcome is that the Chairman agrees to call an extraordinary meeting to take a decision on some urgent matter.
For the sake of completeness, there is also a range of matters on which decisions can be taken without an agenda item or prior notice, such as the creation of a committee or the appointment of committee members. Or the rearrangement of the agenda.
But the clerk is always right if they remind the council that a decision on a substantive matter cannot be taken unless it relates to an agenda item. And philkemp is quite right to point out that the agenda must give a clear indication of the kind of decision that may be taken.