My AC compressor has been grinding on occasion, usually if the AC is on just after I start the car. I opened the hood and had a geez, and noticed when the AC is on, the outer part spins, and when it grinds it stops.

Is the clutch (not to sure if there's a clutch in it) screwing up? Does it need a regas? Any ideas?

Your bearings might be on the way out - they are lubricated by oil in the gas charge.
If you grab the outer plate (the compressor clutch), and manually rotate it, it might give you an idea on how the bearings and internals feel.

How hard is the bearing to replace if this is the problem?

either the clutch bearing on the front has s**t itself, or the compressor has s**t itself. That is a fact.
Keep using it, it will in the not too far future seize up and become completly useless.

If its the clutch that has s**t itself then you need to find out what has made it stop working. Usually a comp fault, constant running of no gas, if no gas then that means no oil, so is running dry, slowing it down, causing the coil in clutch to burn out etc etc lol.

Clutches sell for around $150+postage and compressors(including clutch) sell for roughly$350 depending on which sanden comp it is(or in fact a sanden at all). All full reco with 12 month warranty, aus wide.

Pm if intrested.

Cheers.