Thank you to Tees. Definitions are underlined in the clues.
Across
1. Pound succeeded Dickensian character, female, past prime (9)
SHEEPFOLD : S(abbrev. for “succeeded”) + HEEP(Uriah, character in Charles Dickens’ novel “David Copperfield”) + F(abbrev. for “female”) + OLD(past the period/years of one’s prime).
Defn: …/enclosure.
6. Closely examine a little pink dress (5)
PROBE : 1st letter of(a little) “pink” + ROBE(a long, loose outer garment/clothing/dress).
9. Proportion shown as harangue avoids extremes (5)
RATIO : “oration”(an formal speech given angrily or forcefully/a harangue) minus its 1st and last letters(avoids extremes).
Defn: … between two quantities.
10. Rustic retreat housing a clergyman (5,4)
RURAL DEAN : RURAL(rustic/characteristic of the countryside, as opposed to the town) + DEN(a retreat/a place where one can be alone) containing(housing) A.
11. Travels with old woman in express: one to hold up train? (10)
BRIDESMAID : [ RIDES(journeys on, say, a horse/travels) plus(with) MA(an informal term for one’s mother or an old woman) ] contained in(in) BID(to say/to express, say, a greeting, as in “to bid welcome”).
Defn: …, the train of a bride’s wedding gown, that is.
12. Green plug first person in Paris should insulate (4)
JADE : AD(short for “advertisement”/promotional material/plug) contained in(… should insulate) JE(in the Parisian language, “I”, the first person pronoun).
Defn: The colour, …
14. African golf club admits wife at noon (7)
RWANDAN : [R AND A](or R&A, abbrev. for the Royal and Ancient Golf Club of St. Andrews in Scotland) containing(admits) W(abbrev. for “wife”) plus(at) N(abbrev. for “noon”).
15. Regular couple in Delft provide current address (7)
EFFENDI : 2nd and 4th letters of(Regular couple in) “Delft” + FEND(to provide/to look after oneself without any help from others) + I(symbol for electrical current, in physics).
Defn: An …/title of nobility in Turkey.
17. Bank invested in soccer tournament to make killing (5,2)
CLEAN UP : LEAN(to tilt sideways/to bank, as with an aircraft making a turn) contained in(invested in) CUP(a soccer tournament in which the winners are awarded a trophy in the form of a cup).
Defn: …/to make a substantial gain or profit.
19. Painter entertained by diplomat is Seurat (7)
MATISSE : Hidden in(entertained by) “diplomat is Seurat“.
Answer: Henri …, French painter:
… and Georges Seurat, another French painter: 
20. Diamonds: diamonds get you murdered! (4)
ICED : ICE(slang for “diamonds”, the precious stones made of carbon) + D(abbrev. for “diamonds”, the suit in a deck of playing cards).
Defn: Slang for …
22. Violin piece by leader strengthened position (10)
BRIDGEHEAD : BRIDGE(the part of a violin over which its strings are stretched) plus(by) HEAD(leader/chief).
Defn: … secured by an army inside enemy territory from which to advance.
25. Left cafe worried about university giving intended result (9)
EFFECTUAL : Anagram of(… worried) LEFT CAFE containing(about) U(abbrev. for “university”).
Defn: …/successful in producing the result aimed for.
26. A medic going round hospital for special case only (2,3)
AD HOC : A + DOC(short for “doctor”, a medic) containing(going round) H(abbrev. for “hospital”).
Defn: …/for a particular purpose, as a necessity.
27. End to regulations in American airspace (5)
SINUS : Last letter of(End to) “regulations” + IN + + US(abbrev. for things American).
Defn: A cavity within a bone or other tissue in the body, which might be called an “air space”.
28. Driers that close when spinning (3-6)
TEA-CLOTHS : Anagram of(… when spinning) THAT CLOSE.
Defn: Cloth … used after washing dishes.
Down
1. Abandon vessel that may sink around Costa Rica (5)
SCRUB : SUB(short for “submarine”, a vessel that, by design, can sink into and operate underwater) containing(around) CR(international country code for Costa Rica).
2. Old stock about to come in for free (9)
EXTRICATE : EX-(prefix signifying “once”/old) + TRITE(stock/hackneyed, as in “stock phrase”) containing(… to come in) CA(abbrev. for “circa”/about/approximately, when used in reference to years/time periods).
3. Legal action going ahead (10)
PROCEEDING : Double defn: 1st: The act of initiating legal action/proceedings.
4. Blade-wielder in leaving San Marino wounded (7)
OARSMAN : Anagram of(… wounded) [ “in” deleted from(leaving) “San Marino“].
Defn: …, ie. one who is wielding/using an oar, of which the blade is the flat part put into the water.
5. Nocturnal creatures in sleeping quarters freeze (7)
DORMICE : DORM(short for “dormitory”/a large room serving as sleeping quarters for a number of people) + ICE(to freeze/to form ice).
6. Preparation initially taken by sick? (4)
PILL : 1st letter of(… initially) “Preparation” plus(taken by) ILL(sick/unwell).
Defn: Medical ….
7. Piano used in old time musical work (5)
OPERA : P(abbrev. for “piano”, musical direction to play softly) contained in(used in) [ O(abbrev. for “old”) + ERA(a long time period) ].
8. Bondsman to have letters dictated for comedian (5,4)
ERNIE WISE : ERNIE(acronym for “Electronic Random Number Indicator Equipment”, used in generating random prize-winning numbers in the UK Premium Bonds Lottery, and also a man’s name, hence “bondsman”) plus(to have) homophone of(… dictated) “Ys”(plural of the last but one letter in the English alphabet).
Defn: …, one half of the comedic duo, Morecambe and Wise.
13. Left field of substandard runners hitting this? (3-3-4)
OFF-THE-WALL : Cryptic defn: For substandard runners, especially marathoners, there is a point in the race when the body gives way, and they are then said to have hit the wall.
Defn: …/oddball/eccentric.
14. Criminals who run insolvent businesses? (9)
RECEIVERS : Double defn: 1st: … who accept or buy stolen goods; and 2nd: Those who are appointed by a court to manage the financial affairs of insolvent/bankrupt businesses/companies.
16. Sunshine modified temperature in shelter (6,3)
NISSEN HUT : Anagram of(… modified) SUNSHINE + T(abbrev. for “temperature”).
18. Turned up to run clubs perhaps as hobby (7)
PURSUIT : Reversal of(Turned) UP plus(to) R(abbrev. for “run”, in cricket scores) + SUIT(an example of which/perhaps in card games is “clubs”).
19. Intend shortly to enclose grey brain part (7)
MEDULLA : “mean”(to intend/aim to serve a purpose) minus its last letter(shortly) containing(to enclose) DULL(grey/lacking brightness).
Answer: Short for “medulla oblongata”, the lowest part of the brain stem.
21. River rising to claim female, small and delicate (5)
ELFIN : Reversal of(… rising, in a down clue) NILE(river in Africa) containing(to claim) F(abbrev. for “female”).
23. Cuts links with space station (5)
DOCKS : Double defn: 1st: … the end of an animal’s, say, a dog’s tail; and 2nd: As when a spacecraft …
24. Page taken from agreements in book before Romans (4)
ACTS : “p”(abbrev. for “page”) deleted from(taken from) “pacts”(formal agreements/deals).
Defn: Book preceding Romans in the New Testament of the Bible.
Always great to have a Tees puzzle on a Sunday
Thanks to him and scchua
This was good fun but it was a shame about the use of two Americanisms – “iced” and “left field”.
Thanks to Tees and to scchua.
Why, exactly, is it a shame?
Very enjoyable puzzle. BRIDESMAIDS, BRIDGEHEAD, OARSMAN and DOCKS my favourites.
RD @2: Of the two you highlight, surely left-field has now been in use long enough to have become accepted as ubiquitous rather than US? In online Chambers, ice is indicated as US criminal slang but left-field as informal, esp US. Does not the esp give Tees some wiggle room?
Very enjoyable throughout. Top slightly more difficult than the bottom.
Favourite 27a.
Many thanks to Tees and scchua
Apologies to setter and blogger as I inadvertently signed off without the customary thanks.
I don’t quite understand the parsing of 13d. The runners are said to ‘hit the wall’ not ‘hit off the wall’. Where does the ‘off’ come from. I can see ‘of’ in the wordplay, but that still leaves me short of an ‘f’.
Otherwise a fun puzzle – thanks Tees – and thanks scchua for the blog.
Sourdough @7: although scchua has defined 13d as a cd, I wonder if the ‘substandard’ might equate with OFF with ‘runners hitting this?’ equating to THE WALL?
This compiler does not habitually leave loose ends. I think OFF is ‘substandard’ and ‘the wall’ is what runners, not necessarily substandard ones, may hit.
I note Collins has LEFT-FIELD (adj) as ‘mainly British informal’. ICE they say is ‘US slang’.
(In the sense of ‘kill’ I mean. Sorry.)
the last plantaganet @9, he wouldn’t normally put ‘avoids’ where he needed ‘avoiding’, either (9a). Perhaps he was having an off day.
There’s been a bit of picking of nits, but I’m with crypticsue @1. Thanks to both
or of + f= substandard?? but what do I know? I had the shelter manufactured by a Japanese car maker, and was wondering how DICE get you murdered?
Thanks Tees for the Sunday fun. I have no nits to pick — my failures were my own doing — EFFENDI was unknown to me and I couldn’t parse my guess of ERNIE WISE. As usual, there many good clues including CLEAN UP, SINUS, and OARSMAN. Thanks scchua for the blog.
James @ #11: depends on your understanding of how ‘as’ can work in any given cryptic context, I guess.
Many thanks to scchua et al. I don’t mind a bit of nit-picking, as long as folks know their stuff. Which in general they don’t 😀
Yes, too hasty