A delightful puzzle this, not easy but reasonably fair. A few clues a bit awkward, perhaps as a result of making them read like the thematic material, and hence be almost as impenetrable as it :-) I had a few parsing queries, but they've all been addressed here, many thanks all.
I saw a few mentions of 13a -- I understand there was a misprint of some kind, which I never saw as I printed from the web site where the correction was presumably already applied, so I imagine the correction resolves the problems, but just for reference I parsed 13a as two words of definition then a revolutionary anagrind followed by fodder of a longer word from which a three letter word, scrambled, is removed, then two letters residue after axing, plus a one-letter abbreviation, all of which can be arranged into the answer. The split between products 19 and 20 is a shade awkward, but works.
The most infuriating aspect of this puzzle for me was the essentially random order of the clues (to make the instruction work out, obvs), so I seemed to spend a lot of time hunting for clues! This was exacerbated by not printing A or D for the across or down where applicable, leaving us to figure out which clue was for which by the respective lengths. I think they could have stretched to saying "16,1D" rather than "16,1", for example, without blowing their first week ink allowance. But this is only a minor gripe. Mostly, thanks, Priest. Nicely challenging start for the year.