Thanks, Prospero, ilippu and castafiore. I now have the missing letters, the six modified words, and the six-letter phrase (although the final two words don't help me). However, I cannot find anything sensible for 4D and 19D. A I said, I assume the first word is a sort of wind (that would fit with the theme). But the second word is less obvious. I thought of a pair of spectacles, but now think it must be a synonym for the third word in the six-word phrase. Am I right?