I think Tic-Tac-Toe would help you, indeed. Let me share how I started, I started learning by,
- Writing arithmetic functions in Clojure; for example, next-prime, prime?, factors, prime-factors, and stuff like that
- Then making those idiomatic as far as I could
- Then ask question here, or anywhere, to improve a particular function
Here is the link of math.arithmetic.
Note: It's missing a decent README. I'll put that later. Actually, I never thought of sharing this because it's so trivial.
[Edited]
You can try 4clojure.
4Clojure is a resource to help fledgling clojurians learn the language through interactive problems.
[Edited, again]
There is also Project Euler, which many people have found useful for precisely this sort of thing.
Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve.