De Maar made a break for the front with six kilometres of the 146.9km Stoke-on-Trent stage left, making the move only four kilometres after a crash.
He raced to the finish alone, with Sep Vanmarcke winning a sprint finish for second place some 15 seconds further down the road.
Cavendish fell away from the leading pack in the hills, and could not stay close enough to retain his overall lead.
Cavendish dropped back to 61st, a result that saw him tumble all the way down to 35th in the overall classification.