Exercism: clean and format a phone number in Clojure
I'm still learning clojure, but I'll contribute what I can.


I don't know if it is prefered or not, but if you wanted to get rid of the let, you could do something like (apply (partial format "(%s) %s-%s") trio) or (apply format (concat '["(%s) %s-%s"] trio)) instead.
Naming seems ok.
No comment.
I don't know.
Your regex for parts is suboptimal.  '.' will pick up any character, but you really only want digits.  Something like "^1?(\d{3})(\d{3})(\d{4})$" would be more accurate.