ive been a bit wary of posting this but i really wouldnt recommend clothesline, its generally too thin,often very scratchy generally made of poor quality material even if you go for nylon,and the knots are likely to in fact will loosen very very easily.
and if you run it through the washer it will soften it a little but it will also make it weaker,fray much more easily and will be too pliable.

may i ask exactly what kind of tie you're looking to practice?