Arithmetic expression simplifier in Clojure

Question

I want of create simplifier of arithmetic expressions in Clojure, and I am new to this language.
So for ex.: in: "2x + 6y - (12 + (5x - 3y)) + 4" simplified: "- 3x + 9y - 8".

So my attempt is to parse expression with regexp into hierarchical vector of nested expressions like this: ["5x-3y" "12 + <?>" "2x + 6y - <?> + 4"] ;; <?> is evaluated item from previous step and then evaluate them in sequence.

I am feeling like it's hack, some advice would be helpful.