Sounds like your compressor is siezed but the pulley still spins freely for the time being. The clutch engages when the settings are on defrost (any temp) or obviously AC. Solutions are either replace the compressor, remove it and buy an AC bypass pulley so that you can retain the same length belt, or get a shorter belt to bypass but thats usually trial and error and can result in several trips to the parts store and / or a thrown belt or two. Me personally, Id just get a nice polished bypass pulley and throw out the paperweight that used to be your compressor. Saves a good amount of weight off the front end. If you use your AC a lot then youll have to bite the bullet and get a new compressor. Good luck, job itself is easy enough just got to make sure you discharge the AC properly.