Title: “Further Diminishment”
Prompt: Which entry in this grid is the hidden sixth theme entry?
Answer: POP ICON at 15-Across
Correct entries: 611 overall, of which 585 were solo solves

To find the hidden sixth theme entry we must first look at the unhidden five:

16-A: [“Don’t try to talk me out of it”] = THAT’S MY DECISION
21-A: [County library or city police department, e.g.] = PUBLIC ENTITY
39-A: [Midwestern town named for the country of origin of many of its founders] = BELGIUM, ILLINOIS. Population 400. Anyone here been?
49-A: [Cover on Graduation Day] = ACADEMIC ROBE
62-A: [One whose absence is required] = PERSONA NON GRATA

Concealed in each of those is a metric prefix, getting smaller as we go. I’m going to steal joon’s paragraph explaining it here because if I write it myself I’ll screw something up:

the theme answers contain, in order, the first five metric prefixes for the negative powers of ten (i.e. the ones less than 1), in order: DECI (10^-1 = one tenth), CENTI (10^-2 = one one-hundredth), MILLI (10^-3 = one one-thousandth), MICRO (10^-6 = one one-millionth), and NANO (10^-9 = one one-billionth). the sixth would have to contain PICO, the prefix for 10^-12 (= one one-trillionth), and there it is in POP ICON. so that’s the answer.

Thankfully the comments solvers submitted with their answers didn’t devolve into a punfest employing other metric prefixes. You all are better than that, and I’m pleased that we’re not desecrating the site here today with this sort of nonsense.

Jerry Miccolis says:

Atto boy, Matt!

Thanks for the encouragement, Jerry! I was pretty pleased with the puzzle, and it means a lot that solvers were as well.

Lirath writes:

I enjoyed the hecti- out of this one

Thank you!

jagoandlitefoot writes:

haven’t been doing many crosswords lately, so now i have a lot oF ‘EM TO catch up on

Appreciate you including the MGWCC in your roster! But can anyone tell me why four solvers referenced a mythical Cafe Mt. Olympus in their notes?

And on a non-metric-prefix-hiding note, lhj writes:

Pico was my granddaughter’s favorite street in Los Angeles when she was 2. When we were on it, she’d sing “Pico Pico Pico” the whole time.

Cute! And I’m guessing there was no Zepto Street.

THIS WEEK’S INSTRUCTIONS:

This week’s contest answer is a 10-letter word.

Solve well, and be not led astray by words intended to deceive.