Question

he left the room after the phone call
The clause analyse is

Answer 1

Clause is a group of words in a sentence that gives complete meaning and sense. It has a subject and verb.

In the given sentence, He left the room is the clause. This sentence  has a subject (He) and a verb (left). Moreover, this sentence delivers a meaning and a complete meaning.

Answer 2

He left the room after the phone call.

The clause in the above sentence is  

“He left the room”

A clause is a group of meaningful words which can itself function as a sentence.

It generally contains a subject and a verb.

In a simple sentence, a single clause can function as the sentence.