yeah an overhual would mean removing the clutch from the engine and dimantling it to see if any of the plates/ parts are worn but that would be a LAST RESORT after checking every other possible problem first. It is something that is best left to an experienced mechanic or well practised enthusiast (because of the number of parts involved) and the biggest part of the cost is usually the labour. A good indication that clutch parts need replacing is when both the cable and/ or pushrod have come to their limits of adjustment. Slip and drag are both signs of a poorly adjusted clutch and will lead to major problems if not dealt with. So make sure the pushrod and cable are set properly before deciding is the clutch needs an overhual.
Angela., February 2008