A phrase whose Head word is a verb, called the main verb. The Head can occur alone or together with one or more auxiliary verbs.
The National Curriculum's definition of a clause as "a special type of phrase whose head is a verb" presents us with a new way of looking at clauses. This may take some getting used to. However, the definition makes sense if you view a clause as a grouping of words in which one word stands out as pivotal, and this is the verb. This idea isn't really any different from regarding a group of words whose pivotal element is a noun as a noun phrase, and a string of words whose main element is an adjective as an adjective phrase.
See also clause.