You cannot define fairness entirely in terms of "That which everyone agrees is 'fair'." This isn't just nonterminating. It isn't just ill-defined if Dennis doesn't believe that 'fair' is "that which everyone agrees is 'fair'". It's actually entirely empty, like the English sentence "This sentence is true."

I don't think the definition based on universal agreement is a particularly clever definition, nevertheless I don't see how it is empty. If there was something that everyone agreed was fair, then such definition would be meaningful and non-empty. It doesn't follow that the definition itself must be fair. It is your demand of fairness of the definition of fairness that makes it self-referential.