Apparently some criminal escaped from Riften . Surprise, surprise. We are to hunt them down and kill them, simple enough. The person you will hunt is only named “Criminal” and starts out just west of Riften. They will travel themselves though, so fast travel to Riften Stables and give chase! You should catch up with them pretty soon, and since they aren’t particularly tough you will kill them in no time.
Report your job complete back to one of the circle members for your reward of 300 Gold coins.