Well-known or deducible mnemonics for:
The Taxonomy hierarchy: (Domain), Kingdom, Phylum, Class, Order, Family, Genus, Species
The six wives of Henry VIII: Catherine of Aragon, Anne Boleyn, Jane Seymour, Anne of Cleves, Catherine Howard, Catherine Parr
The first few digits of pi: 3.1415926
The planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
The Mohs mineral scale of hardness: Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond
The Great Lakes (in no particular order): Huron, Ontario, Michigan, Superior, Erie
The Electromagnetic Spectrum: Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-ray, Gamma rays
The Color Spectrum: Red, Orange, Yellow, Green, Blue, Indigo, Violet
are given as across clues in order as:
King Philip Came Over For Good Soup
All Boys Should Come Home Please
How I like a drink? Alcoholic, of course!
My Very Elegant Mother Just Served Us Noodles
Tall Girls Can Find And Other Queer Things Can Do
HOMES
Raging Martians Invade Venus Using X-ray Guns
Richard Of York Gave Battle In Vain
and from each category on word was in the meta as a grid entry. They are:
CLASS
ARAGON
TWO
EARTH
GYPSUM
ONTARIO
RADIO
YELLOW
the first letters of which spell the meta answer: CATEGORY