My 2003 Subabru Forester AC Compressor sems to have seized. Is it merely the clutch or something inside the compressor?

Honestly, based on your description- I'd replace the entire compressor. They aren't too expensive... I just replaced on for $200.00

couple quick checks you can make - test for voltage at the plug - if you are getting voltage, then chances are the climate control is trying to turn it on. If you are not, then i'd start looking for an electrical problem - that could range from a fuse to an issue with the climate control/computer. if there is, then WITH THE ENGINE OFF tap the clutch a few times lightly with a heavy wrench or hammer, then start the motor and see if it "clicks" in - you can also listen for the click to see if it is engaging when the engine is running normally - if you hear a click and see it spin and stop spinning on a regular basis, chances are there is a problem either with the charge or with the compressor itself. if it is NOT clicking into place, but receiving voltage, then you may be able to replace just the clutch - if you can source the part. replacing the compressor should be done by a professional shop, as failing to pull a vacuum and removing all traces of aftermath of the burnout will best case significantly shorten the lifespan and efficiency of the replacement compressor, worst case cause another failure in a short period of time. additionally, if there is a charge issue, then the underlying cause of the likely leak needs to be addressed or you are wasting your money on the compressor.