if you are optimizing for a certain keyphrase x y, but then you find the search count for x y is zero but for y is 10,000. do you 1) optimize for y, even though it's quite competitive or 2) optimize for xy and get to be #1 but with no clicks or is there a better solution?
Don't optimise for a keyword if nobody searches for it. Theres no point. If your site sells 'y' then go for it.
If there are 10k queries for y and OV shows 0 (or doesn't show you) for x y, then I doubt that means nobody searches for it. If you have thought of it, chances are that someone else has too. I don't know that this means x y will be your "money maker" but, your optimization should include your main keyword and also other related terms. It would be natural for a site about y to have links pointing to it for any number of y related terms, right? Think about that and I think you will have your answer.
I would choose "y" instead of xy that has zero search count. It won't matter if your on top rank with "xy" because no one is doing the search