Boy, this sure made me chuckle when I figured it out. "Ex Marx despot." Ha! This is my favorite theme in quite some time. And I love all the theme answers, too. EXPOSTFACTO (17A: Retroactively, at law) is some nice Latin, MARXANDLENIN (29A: Red giants) has a fantastic clue, and DESPOTICALLY (44A: How Ivan the Terrible ruled) took me forever, but I very much enjoyed it once I got it. And to top it off, we've got bonus theme material with two "pirate" clues in the Downs (BUCCANEER & OLDSEADOG) and a final parting shot with ARRR (67A: Pirate's interjection). Very nice.
And with all that one might expect to encounter a few oddities, but the damage isn't too bad, really. I've never heard of the PETREL (43A: Long-winged seabird) (almost a 7-foot wingspan!), but maybe birders (especially those in the southern hemisphere) have. I didn't love UNS (61D: Little ____), ATS (39A: @@@), or DELA (57A: Hip-hop trio ____ Soul), but on the other hand, I did enjoy the odd past tenses MEANT and SPELT, REFLEX (6D: Involuntary action) and GROVEL (10D: Abase oneself) are good words, and MAKEPAR (29D: Finish a hole between a birdie and a bogey) makes me want to go out and play a round of disc golf! Additionally, 38A: "The ____ on you!" (classic gag line) (YOLKS) made me smile.
1A: Jazz legend ____ James (ETTA) gets a C. And I think the team for Hilary's presidential BID should maybe consider using "One no-trump" as a slogan. Heh.