I think “too clever” often equates to “too short.” There is real intellectual pleasure in figuring out how to express an algorithm in as few lines as possible. I personally love doing this. And, usually, shorter code is better code. But I have seen many cases where people take this too far. People reduce their code so much that it becomes incomprehensible. (No doubt I have been guilty of this.) So to your question of what is “too clever” I would say that I would prefer 10 lines of readable code to 1 line of unreadable code.