How do you get the clutch arm pin out and a new one in? I must be missing something obvious.

Not sure if you're referring to the pivot point of the are or where the cable attaches to the arm. A brief search on here suggests you can drift the pin out, but again not sure if they're referring to same pin as you are.

It drifts out but notorious for not fitting back in well.
You can use some lock tight but I just tap and use a shouldered bolt