Excerpt wanted between post title and post thumbnail

On https://snag.gy/lFz6QC.jpg you can see how Twenty Sixteen looks.
This shows the post title, THE POST EXCERPT, then the post thumbnail. The post content then follows.

Twenty Seventeen does not provide this – see https://snag.gy/bdn6LH.jpg
You can clearly see the title is followed only by the post thumbnail. The post content then follows. There is no excerpt.

How do I add this excerpt function back in please to the same place as where it was used in Twenty Sixteen? i.e. just below the post title/header. Thanks.