Just finished!
I had the whole right side solved but only half the left side with letters entered in the grid apart from the clashes. For the life of me, I couldn't understand the theme as the four clashing letters of each word seemed to bear no relation to each other. I even wrote them out. Finally, the theme leapt out at me and with the theme in hand the thematic word was now obvious and it helped me solve some of the remaining clues on the left side as it narrowed the possibilities. Definitely one of those that makes you feel silly after you finally see it.
My last ones in were 23 and then 22. I'm assuming that for 22 it is due to some blue slang that is not used in my part of the English speaking world. I haven't really bothered to figure out why 23 works but the definition is obvious.