Hi there,
I having trouble understanding why “Clip content” changes the look of drop-shadows (unclipping generally darkens the shadow). Here’s a screenshot of the same simple frame with the same drop shadow applied. Left is clipped, right is unclipped.
I found that the SPREAD only became active for me when I added a background color to the shape I wanted to add the spread to. Also, the parent shape of the one you want to add the spread to has to have the Clip Content enabled.