Great fun from Monk this morning.
A good mix of easier and harder clues. There’s a nice symmetry in the first and last of the across clues. Many thanks to Monk!

ACROSS
1. Dismal vitamin deficiency (5)
BLACK
B (vitamin) + LACK (deficiency)
4. Iguana wandering around estate, say, in Central American republic (9)
NICARAGUA
IGUANA* (*wandering) around CAR (estate, say)
9. Arced ornament from Turkey linked to quite eccentric artist (9)
TRIQUETRA
TR (Turkey, ISO code) linked to QUITE* (*eccentric) + RA (artist)
10. Irish currant bread behind grocer’s last jams (5)
BRACK
BACK (behind); [groce]R (‘s last) jams
11. Nail catches tail of boxer dog (5)
TRACK
TACK (nail) catches [boxe]R (tail of)
12. Mostly, sheepdog is about working conflict (9)
COLLISION
COLLI[e] (sheepdog, mostly) + IS< (<about) + ON (working)
13. Drink sent round European castle, one tending queen and her entourage? (9)
BEEKEEPER
BEER (drink) sent round (E (European) + KEEP (castle))
15. Obstructive cartel holds back hint (5)
TRACE
[obstructiv]E CART[el]< (holds, <back)
16. Instant semi-sweet coffee cleared out (5)
TRICE
TRI[fle] (sweet, semi) + C[offe]E (cleared out)
18. Weakened elbow broken with a bat (2,1,3,3)
AT A LOW EBB
(ELBOW with A BAT)* (*broken)
21. Understand focus of chorus in vulgar opera (9)
RUDDIGORE
(DIG (understand, slang) + [ch]OR[us] (focus of)) in RUDE (vulgar)
Ruddigore is a Gilbert & Sullivan opera
22. Dull ceremony’s on time (5)
TRITE
RITE (ceremony) on T (time)
24. Decree absolute at last appearing on legal document (5)
WRITE
[absolut]E (at last) appearing on WRIT (legal document)
25. Taking notice of Liberal voters’ ban (9)
OBSERVANT
(VOTERS’ BAN)* (*liberal)
27. Cast hinge secures firm, round temporary strongbox (5,4)
NIGHT SAFE
HINGE* (*cast) secures FAST< (firm, <round)
28. Solvers and setter eating buffet pasty (5)
WHITE
WE (solvers and setter) eating HIT (buffet)
DOWN
1. City nevertheless recalled cleaning equipment (7)
BATHTUB
BATH (city) + BUT< (nevertheless, <recalled)
2. Soul music’s origin embedded in uncovered crazes (5)
ANIMA
M[usic] (‘s origin) embedded in [m]ANIA[s] (crazes, uncovered)
3. Joint pair of kings confused with clue about knight (7)
KNUCKLE
((K + K (pair of kings) with CLUE)* (*confused)) about N (knight)
4. Final fixture in tennis court? (3)
NET
Double definition
‘Final’ in the sense of salary say – the final amount after deductions
5. Sycophant left with an ecstasy, one that relieves uphill struggles? (7,4)
CRAWLER LANE
CRAWLER (sycophant) + L (left) with AN + E (ecstasy)
6. Again put up pound in blighted Beirut (7)
REBUILT
L (pound) in BEIRUT* (*blighted)
7. Standing up, old French comedian cracks weighty sink (9)
GRAVITATE
TATI< (old French comedian, <standing up) cracks GRAVE (weighty)
8. Sceptically request social function should ignore Democrat (7)
ASKANCE
ASK (request) + [d]ANCE (social function, should ignore D (democrat))
12. Cut into poached, pickled American squid etc (11)
CEPHALOPODA
LOP (cut) into POACHED* (*pickled) + A (American)
14. Be set up over murder, the ultimate in appalling sin (9)
EVILDOING
LIVE< (be, <set up) over DO IN (murder) + [appallin]G (the ultimate in)
16. Set play during success (5-2)
THROW-IN
THRO (during, literary ‘through‘) + WIN (success)
17. Pressing information stops departure (7)
EXIGENT
GEN (information) stops EXIT (departure)
19. Matured beyond extravagant wife that’s good at heart (7)
OUTGREW
(OUTRE (extravagant) + W (wife)) that’s G (good) at heart
20. Whisper, sombre, a theatricism in part (7)
BREATHE
[som]BRE A THE[atricism] (in part)
23. Starts to interview refugee, asking questions in native tongue (5)
IRAQI
I[nterview] R[efugee] A[sking] Q[uestions] I[n] (starts to)
26. Petition for withdrawing half of London station (3)
SUE
EUS[ton]< (London station, half, <withdrawing)
Yes good fun and not too hard for a Monk. I didn’t know the ‘ornament’ sense of TRIQUETRA but am familiar with it in the singular as one of the bones in the wrist. I liked CRAWLER LANE, a part of the road I probably use too much!
Sorry, you may have been referring to this anyway, but there’s a bit more than just the symmetry of the first and last across clues; sequential links as well as symmetry perhaps?
Thanks to Monk and Oriel
He turns black into white one letter at a time via brack, track, trace, trice, trite
…and write.
Otherwise this was a bit of a battle with several NHOs and unparsables (by me anyway). After letting us off lightly last week it feels like the ft is making us work harder this week.
Liked trace, observant.
Didn’t love triquetra (eh?) trice, sue (both use half words with umpteen possible fodder words).
Like yesterday it’s probably good for me but then so is hard work and exercise.
Lots of fun, I came up short on a few but that’s OK. I liked Crawler Lane, Cephalopoda, At A Low Ebb (like my solving skills recently!) and Knuckle.
Thanks Monk and Oriel.
I spotted that as well, James @2. Very clever, not that that’s unusual for Monk.
I know this as a “word ladder,” a game that often appears in word puzzle collections, and a fun bonus in this puzzle.
Not too hard, although “TRI[FLE]” for “semi-sweet” felt like a guess, and “EUS[TON]” I had to look up (as I often do for UK landmarks). TRIQUETRA and RUDDIGORE were NHOs, but guessable from the wordplay.
Thanks everyone on the inner workings of this
Filling the grid in a Monk puzzle is not usually quite enough!
Great fun, as you said, Oriel. I’m happy to learn words like TRIQUETRA just as long as they’re fairly clued, as here.
I didn’t even notice BLACK and WHITE never mind the progression. Must try to be more observant!
Thanks both and thanks to those who spotted the word ladder.
Too late to add anything but praise. Terrific puzzle.
Thanks Monk for a superb crossword . Even though I missed the word ladder I thoroughly enjoyed this. It’s fun solving for ‘odd’ words from the wordplay; TRIQUETRA, BRACK, RUDDIGORE, & CEPHALOPODA fit that description for me. Thanks Oriel for the blog.
Thanks very much Oriel for your trademark-forensic blog, and to all for the positive comments. Also to KenMac for swapping the originally posted grid for the polychrome one nerdishly processed via cell-RGB editing in Crossword Compiler.
Glad to see that an unknown word or two given by accurate clueing doesn’t launch an avalanche of hissy fits chez FT crowd; if it’s any consolation, I didn’t know the entry at 9ac until about Y2K, when Mrs Monk designed a “Triquetra Garden” that, ahem, won the student gold medal at the Harrogate Flower Show.
Prob worth noting that clueing so many relatively short (and, by the Nina, similar) entries in the same puzzle is not that easy given that non-repetition of clueing devices is always the order of the day.
All solved without too much trouble, although we failed to spot the word ladder. TRIQUETRA was a new word to us, although we’d often seen a triquetra without knowing what it was called.
Thanks, Monk and Oriel.
They speak Arabic in Iraq, no? Other than that quibble, good fun. I didn’t catch the word ladder, but that’s what comes from solving in two sessions four hours apart. TRIQUETRA was a jorum for me, and as others have said, so clearly clued that the word could be constructed from the instructions, kind of like a cabinet from Ikea.
Thanks Monk and Oriel
23dn: Chambers 2016 p 804 has “Iraqi n a native of Iraq; the form of Arabic spoken in Iraq.” Similar definitions appear in ODE 2010 p 922 and the Concise Oxford 2011 p 749.
Thanks, PB–Merriam-Webster only has the people, not the language. (Though to be fair, this is like calling “American” (“the form of English spoken in the USA”) a language.)