The last couple in were difficult for me. Thanks to Anto. Definitions are underlined in the clues.
Across
1 Moron had misinterpreted doctor’s prime directive (2,2,4)
DO NO HARM : Anagram of(… misinterpreted) MORON HAD.
Defn: One of the principal precepts/prime directive in the Hippocratic Oath taken by doctors.
5 Sex club opening in town upsets spiritual leaders (6)
COITUS : 1st letters, respectively, of(… leaders) “club opening in town upsets spiritual“.
9 Standard saddle regularly found in old fort (8)
STOCKADE : STOCK(standard/off the shelf/common type) + 2nd, 4th and 6th letters of(… regularly) “saddle“.
10 Detector has section replaced with one that’s older (6)
SENIOR : “sensor”(a device that detects the presence of, usually, something in the physical environment, say, smoke) with its second “s”(abbrev. for “section”) replaced by(has … replaced with) I(Roman numeral for “one”).
12 Family matters can help doctors diagnose health problems (5,6)
BLOOD COUNTS : BLOOD(relationship through being part of the same descent/family) + COUNTS(matters/has an impact on).
Defn: That which …
15 Accept offer to go into A&E (5)
ABIDE : BID(an offer, say, in response to a tender) contained in(to go into) A&E.
Defn: … and put up with.
17 Unconventional air that’s missing A flat (9)
IRREGULAR : “air” minus(that’s missing) “A” + REGULAR(without irregularities/even/flat).
18 A nightmare location for Elstree, hosting movie opening before time (3,6)
ELM STREET : ELSTREE containing(hosting) 1st letter of(… opening) “movie” plus(before) T(abbrev. for “time”). Reference is to this nightmare movie (not made at Elstree Studios):

19 Clothing from years ago that’s partly making a comeback? (5)
TOGAS : Hidden in(… partly) reversal of(… making a comeback) “years ago that’s“.
Defn: … of the ancient Romans, many years ago.
20 Put together talk about Republican case … (11)
ORCHESTRATE : ORATE(to talk/to give a speech) containing(about) [ R(abbrev. for a member of the Republican Party) + CHEST(a case/a large box).
24 … sounding annoyed as it came to a head (6)
PEAKED : Homophone of(sounding) “piqued”(annoyed/irritated).
Defn: …/reached the highest point or a maximum value.
25 One panics, shoving weapon into a tip (8)
ALARMIST : ARM(a weapon) contained in(shoving … into) [A + LIST(to tip/to lean to one side) ].
Defn: …/causes panic in other people.
26 Come together at Oval for dessert (6)
GELATO : GEL(to come together/to take definite shape or begin to work well, said of eg. a venture) + AT + O(abbrev. for “Oval”).
27 Second victory for love rat (3-5)
TWO-TIMER : Cryptic defn: A rat/a despicable character in matters of love, who may be said to have scored a second victory/got another partner at the same time.
I’m not absolutely sure of this clue’s description. I tried to fit in a double definition, but somehow “second victory” doesn’t equate to “two-timer”, the former being the win itself and the latter the winner.
Down
1 British libertines put on record they will slow things down (4,6)
DISC BRAKES : B(abbrev. for “British”) + RAKES(libertines/roues) placed below(… put on, in a down clue) DISC(a record/a recording in the form of a, well, disc).

2 Creative use of some lingos provides fresh things to say (10)
NEOLOGISMS : Anagram of(Creative use of)SOME LINGOS. Good surface.
3 Centres of this bakery product become raised (5)
HIKED : Middle 2, 1, 1 letters, respectively, of(Centres of) “this bakery product“.
Defn: …, as with a hemline, say – here it’s hiked up to there:

4 Broadcasting code in Israel prohibits communication (5,7)
RADIO SILENCE : Anagram of(Broadcasting) CODE IN ISRAEL.
Defn: That which …
6 Management blunder created complete spectacle (9)
OVERSIGHT : OVER(complete/finished) + SIGHT(a spectacle).
7 Poor reason to have king deposed (4)
THIN : “think”(to reason/to work out in the mind) minus(to have … deposed) “k”(abbrev. for “king”).
Defn: …/lacking substance or quality.
8 Intelligent assistant on the phone gets flower sent up (4)
SIRI : Reversal of(… sent up, in a down clue) IRIS(a colourful flower).
Defn: …, specifically the Apple iPhone, that answers and performs various commands in response to voice queries.
11 Widely applied code that can’t be read (9,3)
UNWRITTEN LAW : Cryptic defn: If it’s unwritten, it can’t be read, refering to, say, a code of conduct.
13 Recreating similar page endlessly is definitely not original (10)
PLAGIARISM : Anagram of(Recreating) [ “similar page” minus its last letter(endlessly) ].
Defn: That which …
14 Potential client? Or maybe just a gold-digger (10)
PROSPECTOR : PROSPECT(a potential/prospective client) + OR.
Defn: One that searches for gold, by maybe/eg. digging in a gold field.
16 Confident type more open to reject answer … (9)
EXTROVERT : “extra”(more/plus) “overt”(open/not concealed) minus(to reject) “a”(abbrev. for “answer”).
21 … that socialist then incorporates in dodgy forecasting method (5)
TAROT : “a”(abbrev. for “answer …”, from the previous clue) contained in(that … then incorporates in) TROT(a socialist, specifically a supporter of Trotskyism, advocated by Leon Trotsky).
Defn: … used by dodgy fortune tellers (a tautology).

22 Format for images created by magistrate for example (1-3)
J-PEG : JP(abbrev. for “Justice of the Peace”, an alternative term for a magistrate in the courts of England and Wales) + EG(abbrev. for “exemplis gratia”/for example).
Defn: Format for compressing electronic image files.
23 Early Christian beneficiary when Peter gets fleeced (4)
PAUL : Cryptic defn: Reference to the phrase “to rob Peter to pay Paul” which could be paraphrased as “to fleece Peter to give to beneficiary, Paul” – doesn’t quite have the proper ring to it.
Defn: An apostle in the 1st Century.
Thanks Anto and scchua
I had a full LHS and empty RHS with no ideas, so I did the Chifonie, then came back to this. I did eventually finish, but couldn’t parse IRREGULAR or TAROT. Favourites were DO NO HARM and PROSPECTOR.
Not happy with SENIOR. “Older” is a comparitive, so should give “more senior” or “senior to”. Could he have used “elder” instead?
muffin, re SENIOR: an alternative defn. could be “one that’s older”, but that would require “one” doing double duty.
Shurely 22 is (4) and not 1,3-its an acronym
Thanks Anto and scchua, very enjoyable. In 6d, ‘management’ and ‘blunder’ are two separate definitions of ‘oversight’.
Thanks both. How Anto has improved! Let’s hope the new Everyman setter gets better too.
Thanks Anto and scchua. Didn’t get the parsing for TAROT till I read this blog! Not very sure about TWO-TIMER still. Still, enjoyable set of clues.
Copmus@3 — I agree that 22dn should be a (4) rather than a (1-3). JPEG (the digital image format established by the Joint Photographic Experts Group) does not have a hyphen, and the fact that people pronounce it ‘jay peg’ doesn’t mean that it needs a hyphen. Its three-character file extension is ‘jpg’ and I always think of these image files as ‘jay pee gees’ myself.
muffin @1 Senior is also a comparative — in Latin, it means “older” and in English it means “having more seniority.” The senior partner is “older” in a metaphorical sense.
I put in EXTRAVERT at 16d last night, which messed up 20a. I looked up the spelling this morning and found an interesting controversy — see here https://blogs.scientificamerican.com/beautiful-minds/the-difference-between-extraversion-and-extroversion/. I quote: ‘Folklore has it that when Carl Jung was once asked which was the correct spelling—ExtrAvert or ExtrOvert—Jung’s secretary wrote back something like, “Dr. Jung says it’s ExtrAverted, because ExtrOverted is just bad latin.” ‘ The O spelling was introduced in an American dictionary and caught on.
There’s an urban legend that someone wrote Jung (the originator of the term) asking which ws right, and he said, “It’s Extravert. The other is just bad Latin.”
I hope I’ve corrected the misspelling of my name.
TWO-TIME: My supplement to the OED gives two definitions:
1. Characterized by something that has happened or been done twice; two-time loser
2. to be unfaithful to (a spouse or lover)
So, perhaps, a DD.
Thanks for that quote Valentine; very glad to know Jung coined the term; always thought it was Eysenck (introversion, extroversion, neuroticism, et seq). As for the spelling, well wot can u du?
PS thanks muffin for suggesting Anto via Chifonie. Agreed, more fun (LOI Paul, slow to switch in fleecing/robbing Peter, d’oh).
SENIOR is certainly a comparative in Latin. And what about “Joe is senior to Jim”?
Roger @13
I suggested “senior to” (as well as “more senior”) @1, but it does need the “to” for the comparison.
Harder than today’s Cryptic, which is surely not the idea for a Quiptic. I didn’t like “second victory” or “PROSPECT” as a client.
The Latin goes senex, senior, senissimus, but all this talk of comparatives is surely beside the point, which is that senior is a synonym of older in at least one commonly used phrase (‘senior citizen’). Everyone knows exactly what that means.
I found this a bit difficult for a Quiptic, but very enjoyable. I was thinking the same thing as Shirl @5: Anto has improved dramatically.
27a still seems problematic to me. I suspect that trenodia @10 has the intended parsing, but I don’t think it works. But other than that and some minor quibbles such as the enumeration in JPEG, the clues were sound and often had amusing surfaces.
I agree with all Ted’s points – and as an iPhone user, am a little annoyed with myself that it took me so flaming long to figure out SIRI. (I thought “flower” meant river, and so wasted a lot of time hunting for a four letter one that would work backwards…) Many thanks to Anto for some classy clues, and to Scchua for the invaluable explanations.
Thanks to scchua and Anto
Re 27a, I took TWO-TIMER to mean two-time winner but I didn’t like it much. I’ve can only recall hearing it in American sport broadcasting and it can mean winner/loser/participant inter alia, so it merits at least a ?
Mostly good stuff though.
JOHN E @16 I’m sure it means “Upper Echelon” in that context.
John E @16
Surely the “senior” in “Senior Citizen” means “old” rather than “older”?
I failed to solve 7d and 8d, and could not parse 23d, 21d, 20a.
I found this difficult for a Quiptic, but enjoyable for the most part. I pity the newbie who tries to do this cryptic crossword. I think they should just drop the term “Quiptic”.
Thanks B+S.
Muffin @20
As far as the state is concerned, there is a specified age (the state pension age) above which you are categorised as a senior citizen (an unofficial euphemism for OAP). The age at which people are deemed to become seniors by some major British service providers (e.g. the railways) is 60, which many people would regard as middle-aged in terms of 21st century adulthood. You have to be older than a particular age chosen by officialdom in order to be officially old.
A difficult crossword, with some groans of dismay after revealing a couple I could not solve, e.g. 5 AC and 21d.
My question regards 16d and 21d. I thought I had read somewhere that the … at the end of a clue and at the start of the next clue really means nothing in a crossword, or did I just imagine that? Seems obvious that it really should mean something.
Martin: AFAIU the dots don’t contribute anything towards solving the clues, but are merely a way to elegantly spread the surface meaning over two clues. Which seems to be the case here (or are you suggesting that here each of the clues did help wothers solving the other?).