Hello!
I've struggled with this one, but have just had a major breakthrough - the title and author - and that's allowed me to finish the bottom half of the grid. I'm still pretty stumped on the top half, though, and a hint for 12A would help. I believe it to be thematic.
Oh, and am I meant to be doing anything clever with the thematic parts of the thematic words? The title of the hidden work would suggest swapping one thing for another, but I don't see that with 23D, for instance. Do I just write the themed word in the 3rd box? And where do the extra letters in 21 go? Are they placed in the 3rd box, too? If so, it just ends up looking rather messy.
Take care,
Suzy