The largest forward of his era at 6'2" and 200 lbs, Clapper was a notably peaceful player who nonetheless was involved in an unusual incident in the 1937 Stanley Cup playoffs against the Montreal Maroons. Highsticking Maroon Dave Trottier twice in the head, referee Clarence Campbell (the future NHL president) called Clapper a profane name, and Clapper knocked the referee to the ice with a single punch. Speculation was heavy that Clapper's punishment would be severe, but Campbell himself pleaded Clapper's case, stating that he felt he had provoked the Bruin into the blow; Clapper received only a $100 fine for the incident.