Trying to keep answers I like you suggested the plus sign?

You can't save one answer but you can save a link to any question. There is a bar with a star, flag, envelope, and plus, Hover over the plus sign within the question and there will be an option to "add to watch list". Once you add a question it will be on your watch list in "my profile". You can remove any entry later under the same menu. The choice will become "remove from watch list".