Depends on what kind of line it is. Is it the vinyl coated? Then, a simple wipe down should suffice. But if it is the old metal style or regular cotton "clotheline rope", after it has been out in the elements unused for this long, it probablly is best to replace it.
My suggestion to would be to replace it as it might be more trouble than it's worth to try to clean the old one.
We used to have a metal wire clothes line. We just ran a rag over the wire until clean and hang em up. Worked good. No marks. If its real dirty just use a cleaner first, then it should be ready to use.
Run a hot soapy wash cloth over it and then a clean one to "rinse". If the wash one is very dirty run it over a shorter distance, rinse, and re-do.
Honestly, I'd just replace it. Nothing worse than hanging out clean clothes, and having them come in stained! I buy my line at the dollar store. I get the nylon coated wire, so I can easily wipe it down.
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