The problem here, Ulysses and Kettledrum, is that the letters given for the clue reading "Sisters favour trills", for example, may well not be the actual letters involved! Just 'answering' the clues produces the letters ?A?C?S rather than ?A?E?S.
You see, K, the instructions with the crossword made it clear that one had to solve the clues in the normal way and then look for a set of consecutive letters in the answers which could be transposed to form the name of a tree. Only after the transformation should the answer be entered.
For example, the answer to 9 across was KAON. There we see the three consecutive letters KAO which can create OAK, so the final entry is OAKN.
Of course, you can't necessarily just write the answer in until you check criss-crossing answers, because you may well have to alter other letters in an answer!
Is that about as clear as mud, would you say?