De Klerk was the organist at St. Joseph’s Church in Haarlem, playing there the Adema organ from the time he was 16 until his death. As the organist of the city of Haarlem, he performed at least 6,400 times, playing a multitude of improvisations. These were the subject of a doctoral thesis in which Gerard Sars concluded: “Improvising as he did renews church music from within so that participants in the liturgy are not alienated.” De Klerk had his own musical language, a moderately modern idiom, with a leaning toward the style of César Franck and Hendrik Andriessen. “De Klerk, who was gifted with absolute pitch and sharp theoretical and practical insight, had a legendary musicality. […] As an improviser, he was world famous among his colleagues, in part thanks to the Haarlem organ competition each summer, which drew countless organists to the city.” De Klerk was self-taught as a composer.