I think it would be good if the definition for the extension http://hl7.org/fhir/StructureDefinition/valueset-concept-definition discussed how any definition given in a value set relates to any definition given in the underlying Code System. I'm sure there's no much we can do computationally here, but we should at least be clear about the intent: e.g .the definition cannot be inconsistent with the code system definition, but it may provide additional clarity based on a known context of use