I interpreted you as meaning a clarification on an answer or on an "solved" question, if you mean a clarification on a new question by a new user (e.g. new user B wants to answer it), then see even further below.
If we did add a clarification field, it would just turn into a comment field, and we'd be back at square one. We should instead encourage these new users to work within the system (as giving them comment abilities has been declined).
That might mean posting a new question asking about the clarification. I see nothing wrong with this, especially for a new user: I'd gladly upvote their well-written question. (Yes, if you have 10k rep, I treat your questions different than if you have 1 rep.)
However, I also include my public email address in my profile, and wouldn't mind a random new user from using it. (If I'm comfortable spending time responding to comments on SO, might as well do that for email, etc. Some users are still hesitant to send "random" email, though.) It's never happened to me, but it'd probably make sense to update SO with that clarification anyway.
If you mean a clarification on a new question by a new user (e.g. new user B wants to answer it):
The new user could wait, as another user with 50 rep will likely ask or the clarification will otherwise be made obvious. Or they could make their best guess at a good answer that attempts to solve what they consider to be the most likely scenario in the question. Upon clarification they can edit or delete their answer to solve the actual problem.