They've done it again. Two possible answers to 10a. Could be practiSe or practiCe. It's a "double-barrelled" clue where 10a is part of two separate proverbs. The first proverb leads to the verb form and the second part leads to the noun form. I just wondered what others are putting? I think I might just draw a diagonal line through the box, dividing it in half from corner to corner, and put S in one half and C in the other. Then they can take their pick.
I was always taught that "s" is the spelling of the verb, and "c" the noun. So it's "practiSe what you preach", not "practiCe what you preach", but "practiCe makes perfect" is correct because that's a noun.