"A gimmick is a novel device or idea designed primarily to attract attention or increase appeal, often with little intrinsic value." I think this fits the bill. To figure out the unchecked square, take a good look at the grid! (By the way, I've gotten feedback from solvers so far that the center square is inscrutable, so a gentle hint do you notice anything about the diagonals and then a blatant reveal E.)
aha, I didn't spot the gimmick, and had FINE/FINES for 23A/23D, so got myself a bit stuck. clever construction! I agree that circling the diagonals would make it easier to work out (I had put an O)
I'm not sure about 19D - I only know this as "WHILE away the hours", where the past tense versuon would be WHILED.
Thanks for solving and for the feedback! (I added some alternate solutions for the 23A/23D crossing per @strom's suggestion. I'll add some in the center square too. I don't think I can add circles now, unfortunately!)
Totally valid! The initial solution for the center square was "E" — and was only a white square at all because of the "Gimmick" theme. The reason it was an "E" was because the seed of the grid was the diagonals of "E" in the whole grid. But several folks said that it wasn't clear (especially without those "E" squares being circled) so I added in some alternate solutions that seemed "natural" — again, mainly leaning into the "gimmick".
Quick, informal appraisal of legitimacy or correctness
10A
Collapses
11A
How sloppy kissers kiss
12A
Dubbed
14A
Hellenic H's
15A
Ascend
16A
Include discreetly in an email, for short
17A
Sound of disapproval
18A
Site with step-by-step guides
20A
Part to play
21A
Send, as payment
23A
Present tense of 23D
24A
Kin
26A
Unemotionality
27A
Discourage
Verticali
1D
Film holders
2D
____ May Clampett of "The Beverly Hillbillies"
3D
"Thx" counterpart
4D
British prep school
5D
Actor Burton of "Roots", "Reading Rainbow", and "Star Trek: TNG"
6D
Quick freehand drawers
7D
Elected sachem of the Wampanoag people known as King Philip (1638–1676)… or a comet about a comet (?? I don't know, I guess you know this or you don't)
8D
Half steps, in music
9D
Styled like Mr. Clean or Kojak or Professor X or Dr. Evil or Gru or Lex Luthor (why are these characters all White men?)
11D
SI unit of magnetic flux equivalent to a volt-second (through Faraday's law of induction)