Definitely more in the spirit of the question

I'm having the same issue. I don't know what the value of n is for the test which times out, but on my machine I can go up to ~3500:

user=> (time (series-sum 3500))
"Elapsed time: 9775.32137 msecs"
"3.76"

I believe the code is right, as it solves a case which the author has provided an answer in a comment here:

user=> (time (series-sum 59))
"Elapsed time: 0.998973 msecs"
"2.40"

Probably a typo, but your fixed test should be:

(is (= (head [5 1]) 5))