I was wondering about 20d as well, so thanks gem94. It would never have occurred to me to try that! The clues are very, very tough but (mostly) meticulously fair. The exception is the wordplay leading to OLE, which is an interjection, not a noun.
I'm limping in as a late finisher. Annoyingly, getting the theme some time ago didn't mean I could then sail through the rest of the clues. I'm still stuck on 13a and 9d, which - as nobody else has asked - will probably be dead easy. That leaves only the parsing of the definition in 7d, viz 'slammer following face-off?'
I don't submit these puzzles but if I don't know an answer I fret about it for three weeks. Yes, I do need to get out more.