I'm glad I wasn't alone finding a couple of the thematic items a bit obscure. The signpost to the database was much appreciated. I liked the way the theme and the clues interacted in this one. Thanks, Vagans.
Thanks, Jack. I did wonder about your hint but it seemed borderline - and the puzzle's been around for a few days now. I posted my comment really just to ward off anyone giving a more generous 'nudge'!
Many thanks Jack Aubrey and Son1ofrover1 for trying to help me over the line. I have, in fact, just stumbled across the hidden word via Google. Thanks again.
Is the answer to 17d in the database being discussed on this thread? I have found an example by Googling both words and the thematic phenomenon but couldn't see it in the database itself.