While second to none in the labours of the ministry, Clarke was a most assiduous scholar. First the classics engaged his especial attention, then the early Christian fathers, and then Oriental writers; Hebrew, Syriac, Arabic, Persian, Sanskrit, and other Eastern tongues, with the literature which they represented, being among the subjects of his study. Natural science was a favourite subject, and he had an interest in what are called the occult sciences. He contributed to the "Eclectic Review" from the date of its establishment in 1804, and rendered much literary assistance to the British and Foreign Bible Society.