You can remove all the 'watched' videos from the playlist. Those that haven't been watched you will have to select videos individually to delete them. Learn more here: https://goo.gl/YTBYQd . Hope this info helps!
@TeamYouTube Hey, can I delete all videos from watch later list? Like, all at once without a couple of hours of clicking? Please help 

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Thanks. Twitter will use this to make your timeline better. UndoUndo
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