Sounds as if the clutch magnet is knackered. Can be replaced if you have a good magnet and a puller:


From these pictures you can see the big round magnet that sits behind the pully. To get it off you take out the middle bolt. Gently prise the clutch off. Remove the circlip. Get a puller to remove the pulley and the magnet sits in front of you. Get a screwdriver and hammer. Put the screw driver in the screw and tap the hammer to shock the screw. Then remove. Repeat four times and the clutch will be in your hands.
You don't have to remove the compressor to do this .. in fact that is a good thing as releasing air-con gas to atmosphere is illegal and carries a hefty fine .. very hefty and sometimes time at Her Majesty's pleasure.
I assume you have put a direct 12v feed to the wire and looked for the clutch moving.