A clause is a phrase that contains a verb and normally a subject. It can be a full sentence by itself or be part of a sentence.
'The rain fell heavily' is an example of a clause that is a full sentence. 'It fell on the land, which quickly flooded' is an example of a clause as parts of a sentence, in this case a relative clause.
In the classroom
Areas that can be focussed on include defining and non-defining relative clauses, conditional clauses, and how to link clauses together in writing.