Project Euler #20 solution in Clojure
Your first question: you could make range return a list of bigints, and reduce over it

(reduce * (range (bigint 1) 101))


your second question: :


you dont have to explicitly use seq, clojure will automatically treat your string as a seq
you dont have to use the full-blown string to number converter, you could for example use int to get the char code: 

(map #(- (int %) (int \0)) "1234")`

for other ways of getting digits of a number, check out this thread