Air Conditioner "clutch"

I have a '97 Cavalier that makes a nasty engine rattle when the air-conditioner/fan/heating unit is turned off, which disappears when these are turned on.  Guy at the garage says its the "clutch" to the unit dragging.

Question is: how serious is this?  I've been driving around with the air-conditioner or heater on for about two weeks.  Is it the kind of thing I can leave another week or two, or until after x-mas, or should it be fixed pronto?