(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.) This required 2 sittings to complete, with a couple of clues in the SE quadrant that were more difficult than expected for an IoS. Thanks to Poins. Definitions are underlined in the clues.
Across
1 Finish filling crack with it (6)
TRENDY : END(the finish) contained in(filling) TRY(a crack;an attempt, as in “take a crack at solving this”).
4 Top general introduced to head of free church (6)
FLEECE : [ LEE(Robert E., Confederate general in the US Civil War) placed after(introduced to) the 1st letter of(head of) “free” ] + CE(abbrev. for the Church of England).
Answer: A jacket or top made from, well, fleece, a warm synthetic fibre.
8 Defect with Miles, Dicky and Henry after start of back-stabbing (7)
BLEMISH : [ Anagram of(…, Dicky) MILES plus(and) H(abbrev. for the Henry, in physics, a unit of electrical inductance) ] placed after(after) the 1st letter of(start of) “back-stabbing“.
9 Hint of residue found on each model bird made of clay (7)
EARTHEN : The 1st letter of(Hint of) “residue” placed after(found on) EA(abbrev. for “each”) + T(the early Ford Motors model of car) + HEN(a female bird).
11 Novel set of beliefs (10)
PERSUASION : Double defn: 1st: … by Jane Austen; and 2nd: …, as in “worshippers of a certain persuasion”.
12 Transported in carriage with tenor from start to finish (4)
RAPT : TRAP(a horse-drawn carriage) with “t”(abbrev. for “tenor”) moved its initial position to last(from start to finish).
13 Rent collector’s first to quit (5)
CLEFT : The 1st letter of(…’s first) “collector” plus(to) LEFT(quit;departed).
14 Sheik oddly neglected to call about the Spanish mercenary (8)
HIRELING : “Sheik” minus its 1st and 3rd letter(oddly neglected) plus(to) RING(to call on the phone) containing(about) EL(Spanish for “the”).
16 Poor editing spoiled gripping opening of novel (8)
INDIGENT : Anagram of(… spoiled) EDITING containing(gripping) the 1st letter of(opening of) “novel“.
18 Illegal activity of gangster not about revenge at the outset (5)
CAPER : Capone(Al, Chicago gangster) minus(not) “on”(about, as in “the lecture was on the subject of …”) + the 1st letter of(… at the outset) “revenge“.
20 Nettle rash at first seen on priest from the east (4)
RILE : The 1st letter of(… at first) “rash” plus(seen on) reversal of(… from the east, in an across clue) ELI(biblical priest).
Defn: …, as a verb.
21 Successfully manages to secure bolt in steering device (10)
HANDLEBARS : HANDLES(successfully manages) containing(to secure) BAR(a bolt used in fastening).
23 Protective screen placed in position around most of royal family (7)
TESTUDO : Reversal of(… around) SET(placed in position) + the 1st four letters of(most of) “Tudor”(English royal family from 1485 to 1603).
Defn: … used by the ancient Roman army.

24 Right to be uncertain about accepting ecologist’s earliest fieldwork (7)
REDOUBT : [ R(abbrev. for “right”) + DOUBT(to be uncertain about) ] containing(accepting) the 1st letter of(…’s earliest) “ecologist“.
Defn: … used as a defence.
25 Bit of hogweed found in uninspiring section of river (6)
THAMES : The 1st letter of(Bit of) “hogweed” contained in(found in) TAME(uninspiring;insipid) + S(abbrev. for “section”).
26 Object to revolutionary’s restless desires (6)
ITCHES : IT(pronoun refering to a non-human or inanimate object) + CHE(Guevara, Argentinian revolutionary)‘S .
Down
1 Tradesman not entirely concealing Dutch accent (5)
TILDE : “tiler”(a tradesman;a skilled worker specialising in laying tiles) minus its last letter(not entirely) containing(concealing) D(abbrev. for “Dutch”).
Defn: A diacritical mark used in the Spanish and Portuguese languages.
2 Solidarity’s leader embraced by distressed seamen all at once (2,5)
EN MASSE : The 1st letter of(…’s leader) “Solidarity” contained in(embraced by) anagram of(distressed) SEAMEN.
Defn: … or all together.
3 Pull apart largely depressing books on the French (9)
DISMANTLE : “dismal”(depressing;gloomy) minus its last letter(largely …) + NT(abbrev. for the New Testament, books in the Bible) placed above(on, in a down clue) LE(French for “the”).
5 Have an inclination to ring rector to find out (5)
LEARN : LEAN(to have an inclination, physically or figuratively) containing(to ring) R(abbrev. for “rector”).
6 Ralph managed without parking close to hospital department entrance (7)
ENTHRAL : Anagram of(… managed) “Ralph” minus(without) “p”(abbrev. for “parking”) placed below(close to, in a down clue) ENT(abbrev. for the Ear, Nose and Throat department in a hospital).
Defn: … or to en-trance;enchant.
7 Watch Frank about to turn up something revealing (3-6)
EYE-OPENER : EYE(to watch) + OPEN(frank;honest) + reversal of(… to turn up, in a down clue) RE(with reference to;about).
10 Using darts primarily to capture small brown bear (9)
WITHSTAND : WITH(using, as in “do it with your hands”) + the 1st letter of(… primarily) “darts” containing(to capture) [ S(abbrev. for “small”) + TAN(a brownish colour).
13 Quite happy to eat at home when exercising self-restraint (9)
CONTINENT : CONTENT(quite happy;satisfied) containing(to eat) IN(at home;indoors).
15 Call to mind even parts of Greek prayer (9)
RECOLLECT : The 2nd and 4th letters(even parts of) “Greek ” + COLLECT(a brief formal prayer).
17 A disinclination to move in time to cover it up (7)
INERTIA : IN + ERA(a long period of time) containing(to cover) reversal of(… up, in a down clue) IT.
19 All the rage to possess my book about bagpipe music (7)
PIBROCH : Reversal of(… about) { HIP(all the rage;fashionable;up to date with the latest) containing(to possess) [ COR(my!;exclamation of surprise) + B(abbrev. for “book”) ] }.
21 Heroin discovered by border guard (5)
HEDGE : H(abbrev. for “heroin” in slang) plus(discovered by) EDGE(the border;the rim).
Defn: A barrier or protection against something; or a guard against future financial loss.
22 Originally pinched from superb cryptic puzzle (5)
REBUS : Anagram of(… cryptic) [the 1st letter of(Originally) “pinched” deleted from(from) “superb” ].
(Please do NOT post hereinbelow any comment relating to the picture quiz. Thank you.)
Certainly “more difficult than expected for an IoS.” I thought 19dn had to be PIBROCH but couldn’t parse it. WITHSTAND was my LOI after resorting to a word finder; the parsing was obvious once I saw it but I would never have got it, probably because I was convinced the definition was “using” and that it ended in “ing”.
Thanks, Poins and scchua.
Too difficult for me to make much headway but fairly clued, so thanks Poins.
I thought the blog was masterly, though (and helpful for this very inexpert solver!)