|Definition - A clause used as a complement, i.e. used to complete the sentence's meaning.|
Example - He said that John would come.
(Here the clause "John would come" completes the meaning of "He said that")
Example Usages -
1. "Let us begin by considering how the function of clausal complements is expressed in Piraha˜ without embedding. English expresses the content of verbs such as “to say,” “to think,” and “to want” as clausal complements (here the use of a subscript s labels the embedded clauses as theory-neutral): “I said that [sJohn will be here],” “I want [syou to come],” “I think [sit’s important].”
(Source: Cultural Constraints on Grammar and Cognition in Pirahã: Another Look at the Design Features of Human Language. Everett, Daniel L. Current Anthropology, Vol 46(4), Aug-Oct 2005, 621-634. (page 628) )