I assumed it was hard headed (even though I can't quite see how that is being 'down to earth') but thanks for the answers to that one.
Ok, I did think of graduate beforehand, so if the 's' is wrong, the clue to the word containing that letter is...
21 Across, Layer of ceramic slabs
I've put 'tiles' if the 's' is wrong (and it should now be an 'r'), what could that be?