My understanding is that a clue should 'read' as sensible English, not stilted, whilst giving all the necessary componenents to find the answer. A touch of humour and misdirection can also contribute to a good surface.
Thank you for help, both, but I am none the wiser. Let us drop the word "smooth"; now, what does the word "surface" mean, in either a crossword clue or a solution, or how it fits in on the grid? This is what I do not understand.
Sorry I can't be of any further help to you here.
Why not ask the bloggers on the sites you have seen,what they mean by the expression. I am sure they will tell you and you can let us know in due course.
Thank you, Bullfrog. So why call the overall appearnce of a clue, a "surface", whatever that means? And would Chambers agree with you? Seems a bit open to debate to me?
When you look at the clue you see the surface -- a simple phrase which on first reading will say one thing to you. What the best compilers do is hide a completely different word or words underneath that surface. The more natural they make that surface phrase and the more they misdirect you in your search for the hidden answer, the more elegant or 'smooth' the surface.
Not sure if there is any debate Celia - Bullfrog is absolutely correct - the "surface" of a clue is its apparent reading - a good surface leads the solver mentally away from the "hidden" meaning.