Clause

clause: a grammatical unit that contains both a subject and a verb. An independent, or main, clause expresses a complete thought and can stand alone as a sentence. A dependent, or subordinate, clause cannot stand alone as a sentence and must be accompanied by an independent clause. In the sample sentence, "Because I practiced hard, my AP scores were high," the independent clause in "my AP scores were high," and the dependent clause is "Because I practiced hard."

"To remove the liability of such misapprehensions, I deem it proper to append the following brief explanation."
                                                              -Narrative of the Life of Frederick Douglass
                                                                             by Frederick Douglass

The clause in this sentence is "I deem it proper to append the following brief explanation." Using the clause instead of a separate sentence links the two parts of the sentence together more strongly, emphasizing that he wants to make everything clear by explaining himself further. If the clause had been made a separate sentence, the Appendix (in which this appears) would seem more like an after thought and a covering of mistakes than a further explanation of what was originally stated.