I don't think we have enough info to diagnose, let alone guess what the problem might be. When you say it clunks, do you mean something that occurs separate from the normal sound when you start to pedal and the pawls engage? If you are in a high gear and start pedaling hard after coasting you will normally get a clunk sound/feel when the pawls engage. We need a better description of what happens, what gear(s) and how long it has been occurring. It's always helpful if possible to switch out a rear wheel temporarily to isolate the problem, but of course be careful to readjust the derailleur.