Nope, I can't see how you get YEAST from that. I can see that the letters of YEAST are in the five lower entries in each pair, but I have no idea how you were supposed to know those were the significant letters.
There are five entries that don't really fit the clues. In this version of the puzzle I marked the clues with asterisks but in the hardest version they are unmarked.
a MAYFLY doesn't have mantis-like appendages, SEND doesn't really mean to signal, there never really were FADs about those stinky foods. To outdo ... well it is to BEST, but BEST doesn't indicate any particle activity that one is BESTing another in and "in an activity" was an essential part of the clue. And RETIRED doesn't mean "fixed and ready to use".
These clues have mistakes and you must "rise above it all" to fix them
If you rise above the middle Y in MA(Y)FLY you have the (NTIS) in deNTISt and if you replace the Y with that you have the correct answer MA(NTIS)FLY. Likewise S(E)ND becomes S(OU)ND, F(A)D becomes F(ETI)D, BE(S)T becomes BE(ATA)T and RE(T)IRED becomes RE(PA)IRED which all fit the clues better.
So what was the "it all" you had to rise above? Y-E-A-S-T
Those rolls of lawn to be laid down; or British insult for people (as in "It was Rodgers, the lodgers, ---------")
15A
Physician's org.
16A
Sound like a dove
17A
One who knows his way around a tooth(+)
18A
Haggard's genre(+)
20A
Insect with forelimbs similar to a mantis*
21A
To signal*
22A
"What's that floral necklace?" "This? This is -----"
23A
Cavea thoracis (pl.)
27A
The first prime minister and president of Ghana
30A
Brian of ambient music
31A
Pasta, wider than spaghetti, narrower than fettuccine
34A
Abominable Snowmen(+)
36A
When you mine for cookies but only find 3/4 of the mother lode. (yeah, I did that)
37A
Chastise
39A
Like the lutefisk craze fifteen years ago, or the current stinky tofu trend, or the obsession with durian in 90s*
40A
Acronym for a chaotic mess
42A
The family in "Nanny and the Professor" (yeah, well, they never heard of you either)
44A
Vietnamese holiday
45A
"------ caveman could do it"
47A
Before birth(+)
49A
Don Novello letter-writing alias: Lazlo ----
53A
To out-do in an activity*
54A
Don't know; don't care(+)
56A
Taro dish for a Hawaiian dance?
59A
Fixed and ready for use again*
61A
Presidential Candidate of 52 that people liked
62A
Establishment for the lodging and entertaining of travelers
63A
Undoes a tightening
64A
Tapered stake or pin
65A
Cartoon Garden Gnome.... what? I can't just make up acronyms? Now you tell me. Um, okay, multinational geoscience technology services company.
66A
Switch labels
67A
Cologne
Down
1D
Important estimates when you are trying to leave
2D
Greek for "Thing said". In philosophy it refers to sentences or propositions. In Christianity, it refers to the sayings of Christ.
3D
Related to the kidneys
4D
Styling (as hair)
5D
Broadband connection using telephone copper wires and where the downstream and upstream bandwidths are identical
6D
Who was Tommy Lee performing as in "Cobb"?
7D
Replicas
8D
Single-celled creature. (There's more to it than that, of course. It is a rhizopod protozoan with lobed pseudopodia, without permanent organelles but "single-celled creature" is good enough for a crossword puzzle)
9D
Sleazy
10D
Play a role
11D
Pro
12D
Plaything
14D
Inflamed swelling of a sebaceous gland at the margin of an eyelid
19D
Signing one commits you to silence
23D
Well, essential a race track lap, I suppose?
24D
Imply
25D
Authors Bagnold and Blyton
26D
Distress call at sea
28D
Ancient Persian kettle-drum similar to timpani
29D
Oryza consumption
31D
Solitary sort
32D
Incensed
33D
Clock in the shape of a ship; or a napkin holder shaped like a ship; for Discworld fans a desert on the Klatchian continent; in our world a glacier and a river in Chile
35D
Juvenile newt
38D
Not allowing the serving of booze
40D
Big name in motor oil
41D
What "Lonesome Dove" can be said to be; or "Gone with the Wind"; or maybe "New York" by Edward Rutherford
43D
"Have a meal at Oprah's restaurant!"
46D
What "By the rivers of Babylon", or another such canticle from the first part of the bible, could be said to be.
48D
Sports League, includes the Celtics and the Cavaliers
50D
Elevator man
51D
When Mercury, Washington, and Helium chant "We're number one", Earth, Jefferson, and Lithium may claim this number. (Remember that MGWCC?)
52D
Serengeti scavenger
54D
Titular character in "The Good Dinosaur"
55D
Menial laborer
56D
With it; in the know (like someone who has heard of "Nanny and the Professor")
57D
Instrument at a 56A, maybe
58D
Journey segment
60D
Public historically black land-grant research university est. in 1891; Home of the hornets