Clear water strongly implies that the pipe is blocked, rather than backed up, yeah. Which in turn means something got into the drain from this side and is blocking it. Could be simple -- plastic bag -- or horrid -- plaster of paris, paint -- or unspeakable -- silicone rubber ball that just fits in the pipe -- but no way to tell unless you've got a plumber's endoscope on hand.

lsanderson's suggestion to plunger or snake first seems practical. I might try taking the drain cover off and fishing around with a hook, first, if you can. Bent coat hanger may tell you how far in the blockage might be. (If you're really lucky it's a plastic container lid under the drain cover, at least if I'm visualizing the correct sort of floor drain.)