22A. What kind of car is/was Herbie? That'll lead you to the 'mild stimulant' (if you say it aloud).
16 A. Your 'e' is wrong.
33 D. Your 'c' is wrong (you need the name of a river that sounds as if you're asking a question).
5 D. Your 't' is wrong - the answer is just an anagram of 'anger', the name of a famous Royal's daughter.
I don't know if you've got wrong letters because you've been trying to fit answers round letters you already have - you have to treat the answers, as detailed in earlier posts here. That way, they do all fit in. 'Certain' is correct and will go in the grid OK.