你的句子是错句.应该去掉but,去掉but后,你的句子就是同位语+定语从句的结构
完整的句子是：Using renewable energy sources is a simple idea,ONE THAT / WHICH we find it hard to put into practice.
one 指代a simple idea 做它的同位语.而that / which 修饰one,是定语从句的连接词.
如果用what,你的句子就要彻底改写了：using renewable energy resources is WHAT we find it hard to put into practice.
（好,你应该懂了吧?）