Not a great solution, but have you tried disabling extensions in Safari preferences, quitting Safari or even rebooting, then turning the clipper on again?
Just installed latest version of Evernote on Mac with latest Safari. On Evernote/Web Clipper page, selecting install results in no action. Unsuccessfully tried finding it in App Store. Saw note about getting latest Evernote version. I did, but same reaction. HELP!