Odd one today. I read through it after golf and lunch and solved nothing, so I put it aside and finished the book I was reading. I picked it up again and everything seemed fairly straightforward...........................except that I haven't parsed 17d
Most of the tree disease covered with mixture (7)
I have a word for "mixture", which includes an anagram of a three letter tree, but that's as far as I've got! Any explanation would be welcome.