Android Device doesn't have a Clock app
Your phone does have clock if there's that app inside system/app as you told. Its not necessary that it must be shown on launcher, though normally clock apps are shown. I suggest you give a try to some third party launcher on store and see if that does some trick. On contrary, you should be able to access clock by tapping the clock in notification panel.

Also, if there's some app inside system/app, its already installed. And apps inside this folder are called system apps. You cant install them like normal apps by simply tapping and pressing on install (or rather they dont need to be installed like normal). You install them by simply pushing their apk file to system/app or system/priv-app depending on type of app and then setting their permission to 755 or rw-r-r with some file manager. So that "install failed" is justified. In other words, app is already installed.