I believe that it means that that clip is currently in use in your open Premiere Elements project.
Thanks for your answer Steve, so my query now is that the clips in question, when dragged to the timeline overlay each other rather than positioning sequentially, and I can't seem to work out why this is...?
Are you dragging from the Project Panel to the Timeline (Timeline View Mode)? If so, the Clips should go to the Video Track 1, and not to other Video Tracks.
Also, what model of camcorder are these videos from? If you've got AVCHD video with 5.1 audio and you try to use it in a Premiere Elements project not set up for 5.1 audio, the video will not go onto track 1.
If that's not the case -- I'd recommend you drag video clips form the Project panel, not the Organizer, to your timeline.