Solution
Clone VI's cannot be edited. You must call the VI from the file directory, this can be done with the shortcut "CTRL + M". This will open the VI which you will be able to edit.
If you do not need clones in your LabVIEW application, then you can turn this off.
1. Open the VI, then to File >> VI Properties >> Category - Execution
2. In this window, uncheck Preallocated clone reentrant execution. Check either Non-reentrant execution or Shared clone reentrant execution
3. Press OK, save and close the clone.
When the SubVI is re opened it will reference the source VI.