This is a common problem that I have now seen on several cars. I have even seen this on a brand new never installed clutch cable for a 140 Vega (same plastic spacer even though the cable is a different length). I suspect that the plastic shrinks as it ages and eventually cracks due to the stress of the cable housing through the center of the spacer. I don't know of any place that sells the plastic bushing/spacer. As for electrical isolation, that is not the intent as the other end of the clutch cable housing is in contact with the metal firewall, not to mention the cable itself is metal to metal on both ends. More than likely it was originally intended to prevent corrosion from galvanic action between the steel clutch cable housing and the aluminum bell housing. I have replaced the failed plastic bushing by getting a long grade "3" 14mm bolt with a length of shank between the bolt head and the threads, cutting off the threads, and drilling an appropriately sized hole ( I believe 5/16" dia) through the center axis of the bolt. I have had this in one of my CV's for about 10 years now, after having three plastic spacers fail. The zink coating on the bolt body has so far prevented corrosion between the new metal spacer and the aluminum bell housing.