It depends. From what you've described, it sounds like the original question was pretty ambiguous, and perhaps the presence of this other answer makes that clear, but if you put a lot of thought and research into your own, and if other people said "that's right" rather than doubting it, I don't think you should delete it.
That's in-line with the whole philosophy that "there are no stupid questions, because if you're asking yourself something, there's a really good chance someone else is also confused on it."
I'd edit it and leave a note at the top. Importantly, though, the note should say why it's wrong. It sounds like you misunderstood the unclear question, so include in this brief note what you misunderstood about it.
You don't want this to be too lengthy, especially since "metadata" of the sort is generally frowned-upon. But I'd leave something.
This serves two primary purposes:
- It means anyone reading the question won't have to go through your same thought-process, because you've already shown what you thought and proved it wrong.
- It means anyone who reads the question the same way you and those seven other people did, will get the answer to the question they thought was being asked, which is really the goal at that point anyway.
Of course, that's all contingent on this being a minor difference. If, in reality, those seven up-votes were just random and you honestly don't know where they came from, and you were just half-asleep so you wrote a really nonsense answer, just delete it. But if you think it adds something to the question to show an alternate interpretation of the question, then leave it.
As for editing versus commenting, I think a comment is probably the "right" place for it, but I tend to like a quick edit at the top with a horizontal rule (
<hr />) following it, just to make sure everyone reads it. This is a nice exception, just in the interest of making sure everyone walks away with good information.