What version Premiere Elements? What computer operating system?
Are you double clicking the file's thumbnail in the project media to open the Preview Window with its Set In and Set Out points?
Then are you setting the Set In Set Out (1 second segment) and dragging it to the Timeline video 1?
What does the clip extend to - beyond the 1 second duration.
The Timeline 1 second clip can be extended to recovery the whole video on the Timeline. That is characteristic of the trims derived from the Preview Window Set In Set Out points.
Consequently, please supply details related to "the clip gets extended".
What are you staying - the trim on the Timeline becomes greater than 1 second duration spontaneously when it hits the Timeline or you have spontaneously generated a clip greater than 1 second that displays as a fast motion clip.
Please give more details. If any questions about the questions, please do not hesitate to ask.