Unfortunately, sorting by a property of an embedded object is not currently supported in Realm. As you mentioned, one way to achieve this would be to use aggregation, but since you are offline, this is not possible.
One possible workaround is to denormalize your data by adding a new property to the Entry object that stores the value of cells.body. You can update this property whenever a new Entry_cells object is added or modified.
With this approach, you can then sort by the new property using the .sorted() method. Here is an example:
This approach does have the disadvantage of increased storage space and potential inconsistency if cells.body and cellsBody are not kept in sync. However, it may be a feasible workaround for your use case.