Thanks, Brendan. I'm sure you'll round up the other letters, the ones you've got will have you looking at the clues either side of them. The 'two separate groups..' was a big help.
There's no superfluous letter there, Brendan. I don't know what the Ten bit means, sorry. The answer is just a usual word for 'thing', where if you change one letter (advanced rather than ordinary') it becomes 'worthless', the entry.
Can you recall if there is an extra letter in 18a? I've got the answer with one of the usual 2 letter abbr. for "books" reversed, but that leaves KO for 'boor', and I can't match it up, even if I try adding a letter.