A clause is a unit of words that contains a subject and a predicate. A clause generally consists of a subject and a finite verb relating to the subject and any other elements, e.g. object. A clause can be a full sentence or a part of a sentence.
When the teacher arrived, the learners stopped talking.
The learners who were sitting near the front stood up.
Reduced relative clause
A relative clause in which the relative pronoun and possibly the auxiliary have been left out, e.g. in the sentences The person I saw on the bus, ('who' is left out); The house hit by the storm ('which was' is left out).