It's a good thing I'd eaten dinner before solving this puzzle.
The theme is revealed at 38A: Hilarious...or a hint to this puzzle's theme (side splitting). The theme answers all describe a particular side (as in side dish), split into two entries.
5A: With 50-Down, steak go-with (baked / potato)
10A: With 68-Across, fish filet go-with (cole / slaw)
33A: With 44-Across, hot sandwich go-with (onion / rings)
1D: With 69-Across, burger go-with (french / fries)
I'm always happy to see multiword clues, and this puzzle had some good ones. 21A: Behind closed doors (in secret)...a gimme/good guess; 47A: Closed tight (sealed up); 54A: Make a ship stop by facing the wind (lay to); 55A: Newscast lead (top story); and 45D: 2003 #2 hit for Lil Jon and the East Side Boyz (Get Low).
Liked the clue for 6D: x, y and z (axes)...much better than referring to the tool for chopping.
Words not often seen in a puzzle (that I remember, anyway) included 25A: Well-founded (tenable), 34A: Like waves on a shoreline (erosive), 51A: White-knuckle (intense), 2D: Treat splendidly (regale), 3D: One saying "I do" (avower), and 48D: Go-getter (dynamo).
40D: The Beeb is seen on it (telly). I have no clue who or what The Beeb is, but I had the T, so telly seemed like a good guess.
Funniest clue/answer...but not really...appears at 60A: Airline rarity, nowadays (meal). That was actually my first thought, before even looking at the grid, but I was sure that wouldn't be the answer. Sadly, though, it's true. We flew all the way to Hawaii...a seven-hour flight...and got a package containing nine mini pretzels. On the way home, we had upgraded to first class, so we got cashews...probably no more than nine of them. I'm not suggesting that airline food was ever that good, but on a flight that long, a little sustenance would have been nice. From now on, I'll pack some nutritious...and filling...snacks.
That's it for tonight's puzzle. Feel free to comment on anything I may have overlooked, or just let us know your favorites.
Here's the grid...
...and I'll see you tomorrow.