Born in London, Clare and his brother Alexander grew up on a hill farm in the Black Mountains of south Wales. They were raised by their mother, who had fallen in love with the mountains and with sheep farming. Clare describes the experience in his first book Running for the Hills in which he sets out to trace the course and causes of his parents' divorce, and recalls the eccentric, romantic and often harsh conditions of his childhood. Running for the Hills was a UK bestseller and was published by Scribner in the US.

Clare was expelled from Malvern College, then educated at the United World College of the Atlantic. He read English at the University of York, where, like many of his peers and friends, he used a variety of drugs, particularly cannabis.

His second book Truant: Notes from the Slippery Slope describes what happened to him and his peers, concentrating on the political and social culture of the 1990s, and seeking to explore and explain 'the mad elephant years' as the book calls them, the period in which young men are most likely to take drugs, get into fights, go to jail and commit suicide. The book follows Clare's attempts to begin a career in provincial journalism when stoned, manic or depressed, in thrall to the writings of the Romantics and inspired by Hunter S. Thompson. After a series of disastrous and eccentric escapades he ends up penniless on the streets of London, with apparent manic depression. The book tells of a kind of redemption through living and working with a crew of drug addicts and alcoholics in a Chelsea pub ("we know we are the bottle-washers in the luxury hotel of the western world", Clare writes) from where he joins the BBC and ... eventually ... gives up cannabis. The book surveys the long-term effect of cannabis on a diverse group of people, and ends with a passionate warning to other, younger users.

In 2009 Clare's third book, "A Single Swallow: Following an epic journey from South Africa to South Wales", was published by Chatto and Windus. It recounts the author's experience of following migrating Barn Swallows (Hirundo Rustica) from their wintering grounds in South Africa to their breeding sites in Britain. As several reviewers pointed out, the book is less concerned with the birds themselves than with the people and places along their way, and the writer's adventures. Traveling over 6000 miles in three months, via Lusaka, Brazzaville and Algiers, Clare encounters - and, mostly, befriends - emerald smugglers, the Peace Corps, environmentalists, ornithologists, spies, soldiers, slave labourers, policemen, con artists, prostitutes, officials (corrupt and otherwise) an international rugby player (Clare attempts to secure him a UK visa) and in Spain two members of the Guardia Civil, who assault him for trespassing on a missile base.

'A Single Swallow' was widely reviewed, with critics praising its spirit of adventure and vivid portraits of contemporary Africa. Other commentators criticized the premise - notably Mark Cocker, writing in The Guardian, who said that following migrating swallows was a wonderful idea, but "there is just one problem - it's impossible."

Clare is the author and editor of Sicily Through Writers' Eyes, an anthology of writings about Sicily, and a contributor to the collections Red City: Marrakech Through Writers' Eyes and Meetings With Remarkable Muslims. His journalism has appeared in The Guardian, The Sunday Times, The Spectator, The New Statesman, The Financial Times, The Sunday Telegraph and Vogue. His writing now appears regularly in "The Daily Telegraph".

Running for the Hills won a Somerset Maugham Award in 2007, was longlisted for the Guardian First Book Award in 2006 and Clare was shortlisted for the Sunday Times Young Writer of the Year Award in 2007.

"A Single Swallow" was longlisted for the Wales Book of the Year 2010 and shortlisted for the Dolman / Authors' Club Travel Book Award.