There is the same kind of relationship between sentence and clause as there is between group and word: the sentence has evolved by expansion outwards from the clause.
- Halliday (1985), a pag.192

A sentence will be defined, in fact, as a clause complex. The clause complex will be the only grammatical unit which we shall recognize above the clause. Hence there will be no need to bring in the term ʻsentenceʼ as a distinct grammatical category. We can use it simply to refer to the ortographic unit that is contained between full stops. This will avoid ambiguity: a sentence is a constituent of writing, while a clause complex is a constituent of grammar.
- Halliday (1985), a pag.193