In 1816 he traveled by carriage to Saint Petersburg and reported on the landscape and the social and cultural life in the north German and Baltic ports, a few years after Napoleon Bonaparte was defeated and distant travel once again became possible. After the sudden death of his father Gerrit de Clercq, he came to lead the family business in 1817. De Clercq married Caroline Boissevain (1799-1879) in 1817, and they had several children, including Gerrit de Clercq, later known as an editor of De Gids. During the grain crisis around 1820 the family firm got into a deep crisis, from which she never fully recovered her former peak position.
Around that time, De Clercq came into contact with the till the Jewish convert Isaäc da Costa and a profound friendship grew up between them, with De Clercq converting to orthodox calvinism under Da Costa's influence. Together with Willem Bilderdijk, Abraham Capadose, Guillaume Groen van Prinsterer, Samuel Iperusz Wiselius and others, they became the leaders of the Réveil.
When in 1824 the NHM was set up De Clercq left the failing family business to become secretary of the NHM. For that reason he established himself in the Hague. There he began attending the Reformed-Walloon churches. When the NHM in 1831 transferred to Amsterdam, he officially went over from Mennonitism to the Walloon Reformed Municipality, choosing out of conviction infant baptism over adult baptism.
As a manager of the NHM, he had much influence on the arising textile industry in Twente. He was influenced by Thomas Ainsworth, involved in the setting up of the industry in the town of Nijverdal. He wanted to act out his beliefs in his work, for instance wanting to employ small-scale industry rather than large factories, because he found that factories were harmful for the clergyman state of the workers. Furthermore he was a supporter of the NHM dividing her orders proportionally over as many producers as possible so as to get the work done for the lowest price.
In his last years De Clercq placed his views further from those of Da Costa and came more and more under the influence of Hermann Friedrich Kohlbrugge. As a result he withdrew more and more from social and cultural life, opting for denying the world as opposed to Da Costa's active involvement in city life. De Clercq passed away suddenly, after months in which he made more depressed impression in town. Some historians blame Kohlbrugge's dominating views for De Clercq's death, whilst others lay the blame on De Clercq's own idealistic and emotional character.
As a poet De Clercq was especially well known for improvisation, able to speak a poem, without preparation, at only a minute's notice. In Amsterdam a street is named after him, as also in different Twentse places.