Hi everyone, i just learned about the api and am wondering right now how to properly access & use it. I know it works via curl, but as a networking noob that´s about where my knowledge on this topic (so far) ends. I also found the documentation with the exact commands, but do not yet know how to use them. Thanks for any help on the topic! The Slate is an amazing device by the way 
Understanding your goals (what you want to do, in specific), would help people to be able to craft an answer.
I want to be able to use the api to change the device´s mac address to a specific one, other than a clone of my device or a random one (the two options you are given within the mac clone settings of the admin panel)
I’d suggest that you not do this over an existing connection using the impacted interface, without considering that it will “upset” the ARP tables and may result in a significant delay in reestablishing the connection, even longer if a TLS handshake needs to occur.
One way to accomplish this would be to use a script (that could be, for example, securely executed over ssh with a command-bound key pair) to either edit /etc/config/network directly, or use UCI to set the “macaddr” option of the interface in question, then call /etc/init.d/network reload. ubus could also be used to make the change.