His interest in lichenology was initiated after seeing an article about lichens and air pollution in the newspaper Tribune de Genève.

Clerc's first scientific publications, completed about this time, were on the use of lichens as bioindicators for air pollution in the Valais region of Switzerland.

Pursuing these studies, he visited the collection of Polish lichenologist Józef Motyka in Lublin, an authority on the genus.

In 1988–1989 he undertook a postdoctoral research fellowship at Duke University, where he worked with William Louis Culberson and Rytas Vilgalys.

Starting in 2000, Clerc was a docent at the faculty of natural science at the University of Geneva, and in 2008, the head curator at the Conservatory.

In addition to Switzerland, Clerc has also prepared regional lichen inventories for areas in Alaska, Canary Islands, France, Italy, and Scandinavia.

Other genera in which Clerc has published taxonomical work include Botryolepraria, Candelariella, Melanohalea, Melaspilea, Menegazzia, Parmelia, Parmelina, Rinodina, Tetramelas, Verrucaria, and Waynea.