It sound like you trying to clip a smart layer to to a shape you could also just mask the smart object layer like the shape. The thing is the object should be the correct Aspect ratio and size for the shape. You do not want the Smart object layer to be empty. You want the smart object to have the same Aspect Ratio and size of the images you will be replacing the smart object content with. You seem to be creating a template.. Templates that use smart object layers to be replace normally use Smart object layers for they want to add perspective rotation and warping to the image for its use in the mockup design. Replacement images must be exactly the same Aspect Ratio and size for the smart object transform and rotation and warping to work correctly for the Smart object Layer recorded object for Transform is not replaced when you use replace content or edit the object and fit the replacement to the object canvas. Free Photoshop Photo Collage and Mockup Toolkit
Thank you so much. It worked out for me. What I did was, made an empty layer. Converted it into a smart object. Opened it into another tab and changed the Canvas Size to that of my shape. Then I clipped it to the shape and used free transform to put the smart object over the shape.
Works perfectly. It was so simple yet took me a while. Thank you so much!