Sum of digits in Clojure
I was confused by your code until reading the comments. You want to change the name of sum-once to sum-digits or digit-sum (pick this one!) or anything like it, since that's what it really does.

Your current sum-digits should become digital-root (that's the term for what you're trying to achieve).


  The digital root (also repeated digital sum) of a non-negative integer
  is the (single digit) value obtained by an iterative process of
  summing digits, on each iteration using the result from the previous
  iteration to compute a digit sum. The process continues until a
  single-digit number is reached.