I am a little unclear. The compressor has a clutch, which will disenge the compressor while allowing the belt from the crancshaft to keep on spinning. If you mean that it is cycling quickly, you are low on charge.

If you mean that the compressor locks up and the belt squeels because the compressor is too much load, then it sounds like a bad compressor bearing, or clutch assembly and will need replacement.

If the bels is spinning over the pully, but is not squealing, it is WAY too lose. Tighten it up, maybe apply some belt dressing if it is still spinning. better yet, replace the belt, cause it is probably so stretched that it is in need of replacement anyway.

Hope this helps you troubleshoot the ac problem a bit.