Strangely, I found this a little easier than usual, but perhaps that's because anything seems easy after the Prize this week. My COD would have to be 4d, because it is such an elegant &lit that conveys all you need to know about the subject.
rainman, I find 4 a truly brilliant clue on several levels, my COD as mentioned above. Fun subtlety: the solution is perhaps best known for a notoriously [last 2 words of clue]!
I found this tough. I’ve got 18D left: I have D?T?/ B?Y. Is the definition covered in hair or steady on. I haven’t parsed 20A which supplies the T nor 7D like several others but it has to be the biochemical surely!