Make a backup of that app, then under the "special features" menu for that app select "send latest backup", "app + data", then "save file locally". You can then change the storage location, which defaults to internal storage (sdcard0), to go to a location on your external SD. You can then uninstall the app (assuming it's safe to do so without bricking your phone). To get it back at a later time, go to the TB menu and select "import backup", then navigate to the location where you put it.
You can also change the backup storage location in TB under preferences so all your backups go to the external SD, this helps save on internal storage space if that's an issue for you. Note however that this is best done when you first install TB, since any existing backups will not be moved to the new location automatically. It will only apply to backups made from that time forward.
When an app has been backed up in TB and then uninstalled, it will appear at the bottom of the app list in TB with the title in slash text (crossed out). You can reinstall it from there.