Clive's research has been in four major areas, resulting in over 200 books and scientific papers
The value of cuticular and epidermal characters in plant taxonomy
One of his publications in this field, Cuticular Characters as an Aid to Plant Taxonomy (1965) was translated into Polish and Russian. This was based on his PhD research at the Natural History Museum, and used the family Combretaceae as the main material. It was there, under the excellent tutelage of A.W. Exell, J.E. Dandy and W.T. Stearn, that he gained his knowledge of standard taxonomic literature and practice.
Taxonomy of neotropical Combretaceae
Following from his PhD studies and supplemented by almost 40 years of examining and describing old and new collections, his studies of the Combretaceae resulted in Flora Neotropica Monograph 107: pp. 1-369. Combretaceae (2010), and floristic accounts for various South and Central American Floras.
Plant biosystematics and cytogenetics, especially in relation to hybridisation
Biosystematic research has involved cytogenetics, hybridisation experiments and, latterly, DNA-based investigations. Major products are Hybridization and the Flora of the British Isles (editor and main author) (1975) and Plant Taxonomy and Biosystematics (university text-book, translated into several languages including Chinese, Greek and Portuguese) (1980, 2nd edition 1989).
Hybrid Flora of the British Isles (with D.A. Pearman & C.D. Preston) (2015), was awarded the Engler Medal in Silver by the International Association for Plant Taxonomy.
Floristic research into the flora of the British Isles
This has resulted in the New Flora of the British Isles, and the detailed study of alien plants that it entailed was covered in Collins New Naturalist 129, pp. 1-626. Alien Plants (with M.J. Crawley) (2015), which was honoured with The Presidents' Award of the Botanical Society of the British Isles and of the Wild Flower Society.