Sounds like you have a phono cable. The extra wire is a grounding wire. You can just. Leave it unconnected if you are using this as a regular interconnect. I wouldn't cut the wire as you could sell it later as a phono cable.
Cardas Hexlink Golden Five : IC with RCA
I recently got these ( used) Cardas Hexlink Golden Five IC with RCA connectors ( 5 mt long ) . They have a loose wire ( about 9 inches Long with Lug type end connector ) joined at each RCA connector . It looks like for earthing or Ground connection. But I am not sure . Can anyone advise me as to what I should do with this loose wire ? Where do I connect it ? If I do not connect it to anywhere , and just leave it loose , will that be still OK ?