I’m almost there, have all the thematic stuff. Don’t have 13 across, Tree’s dead root, one twisted (6)
I believe the clue has to lose an N. (leaving oe, which officially is a word!).
Usual single letter for "dead" followed by 3 letter "root" (Latin abbr.) + "one" minus the extra N - the last two words are reversed - definition is "tree".
Ok, thanks again, finished - on a Thursday evening.
But wow that (13a), plus 1d, 7d and several others make this the toughest IQ/Listener I have seen in years.