Clarke's reputation took him to India to study the world's largest harrier roosts for the Bombay Natural History Society.[1] He subsequently worked on a project that successfully reintroduced red kites to several English regions.[1][2]
Clarke's first book, Harriers of the British Isles, appeared in 1990.[1][2] It was followed by The Marsh Harrier (1995) and Montagu's Harrier (1996).[1][4] He was co-editor of Biology and Conservation of Small Falcons in 1993 and during the final months before his early death he worked on the second edition of The Hen Harrier, adding to the first edition written by Watson.[1][2]
Clarke was treasurer of the British Ornithologists' Union (2000–06).[1] He also looked after the accounts of the Society of Wildlife Artists (SWLA), whose then president, Bruce Pearson, was among his friends.[1] His collection of more than 200 works of wildlife art formed part of a SWLA exhibition staged at the Mall Galleries, London in 2007.[1][2]