how to clear contents off downloads folder?

Read the articles : Safari for Mac: Download items from the web using Safari

: Safari for Mac: If Safari can’t download an item from the web

If the items are successfully downloaded , then open finder click on download folder select the items ( command + A ) , to delete ( command + delete ) they will be moved into the trash .

Restart the machine and empty the trash .

Open Finder Preferences : Sidebar, and ensure that Downloads is checked under Favorites.

Open a new Finder window, and click Downloads under Favorites. Select multiple items to delete by pressing, and holding the command key, while selecting multiple items. Then, right/control-click, or two finger tap (if trackpad enabled), and from the secondary menu, choose Move to Trash. Empty your Trash.

If on the otherhand, you were referring instead, to the Downloads folder alias icon that is adjacent to the Trash icon in the Dock, there is no facility for cleaning up that list of download items. As files are removed from the Downloads folder in Finder, they will also be removed from this Dock Downloads list.