(Please click here for this same blog but with a picture quiz added. Please do NOT post hereinbelow any comment relating to the picture quiz. Thank you.) Thanks to Daedalus for an enjoyable puzzle, just a bit tricky in parts. Definitions are underlined in the clues.
Across
1 King Edward prepared? Too apt! (6,6)
MASHED POTATO : Reverse clue: Anagram of(MASHED) POTATO = Too apt.
Defn: A preparation you could get using a King Edward potato.
9 Prince Oscar is all about getting things just so (9)
PRECISION : Anagram of(… all about) [ PRINCE + O(letter represented by “Oscar” in the phonetic alphabet) + IS ].
10 Sacred word such as amen‘s first and last (5)
OMEGA : OM(in Hinduism and Buddhism, a sacred syllable used as a mantra) + EG(abbrev. for “exempli gratia”;for example;such as) + the 1st letter of(…’s first) “amen“.
Defn: … of a series.
11 Spot lion and unicorn heading inside to make deal (4,3)
DOLE OUT : DOT(a spot) + LEO(from Latin, the name for a lion, as used in children’s tales, fables, etc.) containing(… inside) the 1st letter of(… heading) “unicorn “.
Defn: To give out shares;to distribute.
12 At first you’re all nervous, giving the Zulu elder a flower (7)
YANGTZE : The 1st letters, respectively, of(At first) “you’re all nervous, giving the Zulu elder “.
Defn: Flow-er;river.
13 Feverish convulsion stopped by shock treatment (6)
HECTIC : HIC(representation of the sound of a hiccup, caused by a spasm;a convulsion of the diaphragm) containing(stopped by;plugged by) ECT(abbrev. for “electroconvulsive therapy”;electric shock treatment).
14 Caught erstwhile setter’s fancy (8)
CONCEIVE : C(abbrev. for “caught”, as in cricket scores) + ONCE(erstwhile;at a time before) + I’VE(“I have” used self-referentially, for “setter has;setter’s”).
Defn: To imagine.
17 Pains besetting ailing hero at Troy (8)
ACHILLES : ACHES(pains) contained in(besetting) ILL(ailing;sick).
18 Teddy maybe comes back to life? (6)
UNDIES : [ UN-(prefix expressing a reversal of an action) DIES ](comes back to life).
Defn: A woman’s one-piece undergarment.
20 Astronomer terribly ill with old age (7)
GALILEO : Anagram of(terribly) [ ILL plus(with) O(abbrev. for “old”) + AGE].
22 Leave rag-ends, keeping what’s modish? (7)
AVERAGE : Hidden in(…, keeping) “Leave rag-ends “.
Cryptic defn: Mode(in statistics, the most frequently occurring value in a set of quantities, and a value that is representative of a set of quantities). -ish(suffix to represent a nearness to;approximately and/or having the characteristics of). Hence “modish”.
Answer: The arithmetic mean of a set of quantities, which could approximate the mode, and like the latter, be representative of a set of quantities.
24 One leaves injured maiden getting better (5)
AMEND : “I”(Roman numeral for “one”) deleted from(leaves) anagram of(injured) “maiden “.
Defn: …, as a verb.
25 A way to understand things during class (9)
INTUITION : IN(during) TUITION(class;the period when a group of students are being tutored).
26 With hollow smile, take silver spear and apprehend immediately (3,2,1,6)
SEE AT A GLANCE : The 1st and last letters of(hollow)”smile” + EAT(to take food) + AG(chemical symbol for the element, silver) + LANCE(a spear).
Down
1 Vehicle idled (5)
MOPED : Double defn: 1st: A motor scooter; and 2nd: To move or act in an aimless way.
2 I investigate the underworld or Hell – shocks me appallingly (8,6)
SHERLOCK HOLMES : Anagram of(… appallingly) OR HELL – SHOCKS ME.
Defn: … of criminals.
3 Study old-time retired leader (9)
EDITORIAL : Reversal of(… retired, in a down clue) [ LAIR(a place to retire to;a study) + O(abbrev. for “old”) + TIDE(used in combination to represent a time or season, as in Christmastide) ].
Defn: The leading article in a newspaper that expresses the opinion of its editor or publishers.
4 Python lacking a python’s core support (6)
PLINTH : “Palin”(Michael, a member of the Monty Python team) minus(lacking) “a ” + the middle 2 letters of(…’s core) “python“.
Defn: The slab supporting a pedestal, column or statue.
5 Sport broadcast just after poet (8)
TENNYSON : Homophone of(… broadcast) “tennis”(a sport) + ON(just;immediately after an event, as in “on arrival, they checked in at the desk”)
6 Letter written long ago with quill? (5)
THORN : Double defn: 1st: A Germanic runic letter used in Old and Middle English, and Old Norse.
7 Invading yuppies infect compound with rigatoni (14)
GENTRIFICATION : Anagram of(… compound …) [INFECT plus(with) RIGATONI].
Defn: The process by which yuppies take over a traditionally working-class area in a city.
8 Dodgy leases in empty buildings with no foundations (8)
BASELESS : Anagram of(Dodgy) LEASES contained in(in) the 1st letter and last letters of(empty) “buildings “.
13 Principal effects of nightcap? (8)
HEADGEAR : HEAD(principal) + GEAR(one’s personal effects, including clothing).
15 Leave to go, having settled up in good humour (9)
CONGENIAL : CONGE(formal permission;leave to depart) + reversal of(… up, in a down clue) LAIN(having settled on;stretched on).
16 In Romeo’s home I start to collect speedwell (8)
VERONICA : VERONA(home of Shakespeare’s Romeo) containing(In …) [I + the 1st letter of(start to) “collect “].

19 Danger failing to finish chant (6)
MANTRA : “mantrap”(a danger to trespassers, poachers, or males) minus
its last letter(failing to finish).
21 Spooner‘s boy left Spain (5)
LADLE : LAD(a boy) + L(abbrev. for “left”) + E(the International Vehicle Registration code for Spain).
Cryptic defn: Something to spoon with, and not the Reverend with the speech idiosyncrasy.
23 I pick winners for Bert’s mate (5)
ERNIE : Double defn: 1st: Acronym for “Electronic Random Number Indicator Equipment”, the machine that picks the winning numbers of Premium Bonds in the UK; and 2nd: … in Sesame Street, the children’s educational TV series.

(Please do NOT post hereinbelow any comment relating to the picture quiz. Thank you.)
The “thorn” character looks very like a letter Y, but stood for “th”. The “Ye” in “Ye Olde Tea Shoppe” and the like uses the thorn (though the owners generally don’t realise this!) and should be pronouned “the”.
Thank Daedalus and scchua.
I did not have time to complete this puzzle today after solving the Everyman, but enjoyed what I managed to do. I especially liked VERONICA, MANTRA and ERNIE! Poor ACHILLES and GALILEO…