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