that's your transmission
it sounds like 2nd isn't engaging which could be as simple as no fluid, or as complicated as a blown clutch or no friction material, both are unfortunately equally likely. That clunking sound is absolutely a transmission problem though, like it does it when starting or when getting high in RPM but still slow where it should be shifting right>
roffs suggestions are right on and very appropriate in general, but for your specific bike and problem it sounds VERY much like trans