He became mostly known for his brilliant, high-keyed, colourful landscapes inspired by Monet and for his interest in painting the figure, mostly academic-influenced and idealized nudes.

In 1907, he travelled to Belgium and Spain, where he studied in Madrid with William Laparra and regularly visited the Prado Museum.

Returning to Montreal in 1908, Clapp brought with him his innovative interpretation of Impressionism, applying it to Canadian subject matter, which received mixed and uncomprehending reactions from critics and the public[1] but at the Art Association of Montreal's Spring Exhibition in 1908 he shared the Jessie Dow Prize with Helen McNicoll.

[3] After exhibiting his work in Montreal in a solo exhibition in 1914, he left Canada and lived in Cuba from 1915 to 1917, then moved to Oakland, California, where he co-founded the Society of Six with Selden Connor Gile, August Gay, Maurice Logan, Louis Siegriest, and Bernard von Eichman, and he wrote their manifesto.

[4] His artwork can be seen at the Smithsonian American Art Museum[2] and the Musée national des beaux-arts du Québec.