Go back to the store for necessary repair.
The "clunk" could be the pedal, though pedals tend to creak as they become loose. You might also have an issue with the bottom bracket or crank. Once you've ridden for any distance on a loose pedal, the crank is typically damaged beyond realistic repair.
If this bike was purchased from a dept store (like walmart), what you're experiencing may be the beginning of a series of failures and problems. The bikes they sell are a mix of inferior components assembled by (usually) unknowledgeable staff.
