The quote is:
"The periodic table is the universal catalog of everything you can drop on your foot. There are some things, such as light, love, logic, and time, that are not in the periodic table. But you can't drop any of those things on your foot."
— Theodore Gray, The Elements
Answers to the clues:
A. Massive pop? Stretch regularly beforehand. — TECTONIC = TONIC (pop) + (s)T(r)E(t)C(h).
B. Without leader, Whigs (around midnight) dumped tea from Boston, leading to subject of July 4 declaration — HIGG'S BOSON = (w)HIG.s around (ni)G(ht) ("midnight") + BOS(t)ON ("dumped tea"), the discovery of Higg's boson was announced on 4 July 2012.
C. Maguire's final Wicked book sees half of Oz come to the defenses of Dorothy and the community at large — EVERYBODY = (maguir)E + VERY ("wicked") + B ("book") + O(z) + D(oroth)Y.
D. Tea left out for more time than necessary — OOLONG = (t)OO LONG; &lit.: oolong leaves are partially oxidized from being left out in the sun
E. Have misgivings halfway through getting to second base with partner, essentially — DOUBT = DOUB(ling) (getting to decond base in baseball, apparently) + (par)T(ner)
F. Trap rodent after dropping candy into antique pot? — OUTHOUSE = (m)OUTH ("trap") + (m)OUSE ("rodent") with m&m, a sweet, cany, missing.
G. Source of refreshment - tops in Indian, Chinese economies — RICE = R(efreshment) + I(ndian) + C(hinese) + E(conomies), &lit.
H. Reader: "Is something the Chinese have often sold to a dictator what's discussed on page 63 in the book?" — EUROPIUM, sounds like "Your opium". Europium's atomic number is 63.
I. Transfer tea leaves from jar at the rear of the ship — GRAFT = GRA(te + a)FT
J. Location of show on Apple TV (or Peacock, initially) following Joey Fatone's origin — ROOFTOP = ROO ("joey", a little kangaroo) + F(atone) + T(v) + O(r) + P(eacock)
K. Dance show without clothing? Very disheartening, and a little fishy — ANCHOVY = (d)ANC(e) (s)HO(w) + V(er)Y
L. Tampa Bay ballplayer changes direction upon encountering defensive unit at midfield, for example — YARD LINE = RAY< (a single Tampa Bay Rays player) + D LINE (defensive line)
M. Mix two lattes for someone who talks a lot — TATTLETALE = (LATTE + LATTE)*
N. Some watch ouroboros for the time it takes to complete a circle — HOUR = hidden in ("some") (watc)H OUR(oboros)
O. The first nation to obtain a bit of independence in East Africa, ultimately — ETHIOPIA = T(he) + I(ndependence) in H.OPI (a native American nation), all inside E + afric(A), &lit.
P. English tea - noon-time delight — ENCHANT = EN (English, ISO abbrev.) + CHA (tea) + N (noon) + T.
Q. Maybe blood pressure almost rebounding after Alex slimmed down - ultimately, because of diet — LEGISLATIVE = (a)LE(x) + VITAL SIG(n)< ("maybe blood pressure almoust rebounding") + (becaus)E; here, the diet is a parliament.
R. One who studies people of other nations, surprisingly in the region around Hawaii — ETHNOHISTORIAN – HI (Hawaii) in (OTHER NATIONS)*
S. One wearing retro style? She's fancy, despite ... — MALIGNANCY = I ("one") inside ("wearing") G.LAM< ("retro style") + Nancy ("she's fancy"). Today I learned that despite is not only a preposition.
T. ... spilling tea over seat, scrubbing last bit of stain — ET CETERA = CE(n)TER (last of stain missing from seat) inside TEA*. If you have trouble to make out the bolded portion in the clue, the definition is "...".
U. Changing fortunes surrounding a vampire — NOSFERATU = FORTUNES* around A.
V. Place where you can spend hours taking breaks in the middle of procrastinating work — THRIFT SHOP = H (hours) + RIFTS (breaks) inside (procras)T(inating) + OP (opus, so "work).
W. Reserved tour of unusual base — SHODDY = SHY ("reserved") touring ODD
I don't think that there is aconnection between the quotation and tea, but tea features in a lot of the clues, as wordplay fodder or as def.
In the comments below, samm82, oAlt and Chris Cudmore gave useful hints to tackle the last missing clues. Do I need to learn more about baseball? Nah, I don't.