"Sounds like" clues can be tricky creatures for both setters and solvers. I see from Chambers that the answer in this case has two common pronunciations. If you use one then the much neater parsing suggested by scorpiojo works - and I guess it's probably what Paul had in mind. But if, like me, you're used to other pronunciation you have to do something more convoluted. Luckily, "rock" works for both as well as providing the surface.