Hello @amardeepmisha,
Thanks for writing in!
You are having this issue because you have a slug conflict. The Clients portfolio page and the slug of your portfolio items is using the very same clients slug. To resolve this, please go to Pro > Portfolio and change Custom URL Slug to client. Keep in mind that you cannot use clients because all the portfolio items are not a sub page of the Portfolio page which is using the clients slug.
Normally, the page and sub page slug relation ship is this:
http://www.example.com/page/
http://www.example.com/page/sub-page/
Be advised the the Clients portfolio is a page and you have several items so there will be paged version of the portfolio page like:
http://www.example.com/Clients/
http://www.example.com/Clients/page-2/
http://www.example.com/Clients/page-3/
And then your portfolio item url should always be:
http://www.example.com/{custom-url-slug}/some-portfolio-item/
Remember that the ‘some portfolio item’ is not a sub page of the Clients page. It is actually a different custom post type which is the portfolio. The Clients page is a normal page that were just being used a container to display the list of the portfolio items.
Hope this makes sense.