The answer is an obscure word. It will help you to know that SMACK can mean "absolutely".
This one's a real stinker. I've got the answer to 1d for example and I know it has to be changed before entry but it seems like a random anagram to me. And this clue makes no sense to me at all:
Messenger's drive to stop working entirely in Scotland (5)
I think the letters A, D, E and P are involved, though not necessarily in that order.