What are some Disney songs with cellos the only background instrument?

Unless it's a recording specifically arranged for solo cello or cello accompaniment and voice (I'm not quite clear if you actually mean a song to be sung that is backed only by cello or not), I don't BELIEVE there is. Not from the original soundtrack. That isn't saying there isn't something out there.

You have things like this:

http://www.halleonard.com/product/viewproduct.acti...

However, those are easy arrangements for a cellist to play, and does not contain lyrics, nor would it particularly serve as background music for a singer since the cello is basically playing a bare melody.

You might have to do some intense searching online depending on what exactly you need or want.