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I've almost finished but I can't quite get it. The ones foxing me are
1D) Desire, perhaps, in passage with Charles. (6 letters, but this one probably has a word removed at some point)
4D) Grime settled on recording medium.
20A) Doctor swallows small narcotic drink and some form of rock.

I have most of the extra letters from the wordplay, but I'm still not quite getting the theme.
Would really appreciate any help - thanks all!

Streetcar, megilp, pillow lava, I think 18 clues for 18 stages

Thanks so much Mr Mournful! I can see a bird theme, but I'm stuck there - this is a tricky one...