Won't Fit on Disc
CloneCD does a 1:1 back-up of movies. It does not use any compression. More than likely the disc to are trying to make a back-up of is a double/dual layer (8.5gb) and you are trying to put it on a DVD+/- RW (4.7gb). To use CloneCD you will have to use a double/dual layer disc.
You can use CloneDVD for that purpose. It does compress the disc. You can also remove trailers, extras, subtitles, etc to get it to an acceptable quality level so playback is not poor.