Have just twigged what 40 across must be! The hyphen comes after the first three letters, I guess. Just leaves the crux - 18 down. I take it the last four letters form a crucial word, but what precedes them I cannot think.
Ironie, if your first letter in 18dn is H then you need to look again at 17ac. If you have 17ac right then if you go to www.onelook.com and search on your four-letter word you will find enlightenment at the bottom of the page, in phrases containing the word.