Thank you to Moo Definitions are underlined in the clues.
Across
1. Secure nice position with multinational (6)
NESTLE : Double defn: 1st: Settle against something comfortable; and 2nd: A … food and drink processing company based in Switzerland.
The second defn. is written “Nestlé”.
4. Admirer of marriage vow? Not yet (8)
IDOLATER : “I DO”(said at the end of marriage vows, to affirm one intends to keep those vows) + LATER(not yet/at a future time).
9. The right footwear for a drug dealer? (5)
BOOTS : Double defn: 2nd: …, cryptically.
… a chain of drug dealers, in fact.
10. SIS signal decoded – it clarifies things (9)
ISINGLASS : Anagram of(… decoded) SIS SIGNAL.

Defn: A kind of gelatin from fish, used in the clarification/fining of wine and beer.
11. See red, finding E on former copper (7)
EXPLODE : E placed after(finding … on) [ EX-(prefix signifying “former”/once, as in “ex-wife”) + PLOD(an informal term for a police officer/copper) ].
Defn: …/go into a sudden rage.
12. Pained expression of famous cricketer one’s caught (7)
GRIMACE : GRACE(W.G., famous English cricketer) containing(… caught) I’M(contraction of “I am”/one’s/one is, with “one” used as a substitute pronoun for “I”).
13. American teacher is lying (4)
ABED : A(abbrev. for “American”) + BED(abbrev. for “Bachelor of Education”, a teacher with such a qualification).
14. Byzantine pope not welcoming northern rival (8)
OPPONENT : Anagram of(Byzantine) POPE NOT containing(welcoming) N(abbrev. for “northern”).
17. Top Gear produced by serious men from the east (8)
SOMBRERO : SOMBRE(serious/solemn) + reversal of(… from the east, in an across clue) OR(abbrev. for “other ranks”/men in the military who are non-commissioned).
Defn: An article of clothing/gear for one’s head/top.
19. The Speaker’s successors resign (4)
CEDE : Homophone of(The Speaker’s) “seed”(archaic term for a person’s descendants/successors).
Defn: …/to give up.
22. Break down on crack (7)
RESOLVE : RE(on/with reference to, a specific matter) + CRACK(to solve/to find a solution to a problem).
Defn: …/to dissect a subject into its components.
24. Traveller revealing fault in German article (7)
DRIFTER : RIFT(a fault/a long break in a rock formation) contained in(in) DER(German for “the”, an article in grammar).
25. The result of eating too much grouse? (9)
BELLYACHE : Double defn: 2nd: …/to complain noisily.
26. Son went to get some dope (5)
SPEED : S(abbrev. for “son”) + PEED(went/passed urine).
Defn: Slang for methamphetamine/”meth”, a recreational drug/…
27. Love song composed about Bill (8)
SERENADE : SERENE(composed/calm) containing(about) AD(short for “advertisement”/a flyer/a bill.
28. During day, social worker sought by police (6)
WANTED : WED(abbrev. for “Wednesday”) containing(During …, …) ANT(a social insect who might be one of the worker caste).
Down
1. He perhaps begs loan uneasily (5,3)
NOBLE GAS : Anagram of(… uneasily) BEGS LOAN.
Defn: Of which, “Helium”, a chemical element with symbol “He”, is an example/perhaps.
2. Way to clamp down on breaking news (4,5)
STOP PRESS : ST(abbrev. for “street”/a thoroughfare/a way) + OPPRESS(to clamp down/to subject to hardship).
Defn: News in a newspaper entered at the last moment just before printing began, or after printing has begun.
The clue could also be read literally since stopping the printing press(es) could be a method of stopping news from breaking/being sent out.
3. Boxer, champion with heart of stone (6)
LISTON : LION(a champion/a hero) containing(with heart of) ST(abbrev. for “stone”, the former British unit of weight).
Answer: “Sonny” …, former heavyweight boxing champion.
5. Making breakfast in prison? (5,8)
DOING PORRIDGE : DOING(making/cooking) PORRIDGE(a dish of oatmeal or other cereal boiled in water or milk, often for breakfast).
Defn: Slang for serving time …, from when porridge used to be served in prisons.
6. Coal left to burn (7)
LIGNITE : L(abbrev. for “left”) + IGNITE(to burn/to burst into flames).
Defn: A soft brownish ….
7. I’m going to secure island crown (5)
TIARA : “TA-RA!”(“Goodbye!”/”I’m going!”) containing(to secure) I(abbrev. for “island”).

8. Disappointed Charlie leaving in emergency (6)
RUSHED : “crushed”(disappointed/felt let down) minus(… leaving) “C”(letter in the phonetic alphabet represented by “Charlie”).
Defn: Describing someone or something leaving in an emergency, as in “a rushed journey to the hospital after the accident”.
10. Reporter’s local suffered, being naive (13)
INEXPERIENCED : Homophone of(Reporter’s) “inn”(a local/a pub near one’s home or office that one frequents) + EXPERIENCED(suffered/lived through).
15. Handling of matter botched by hospital department (9)
TREATMENT : Anagram of(… botched) matter plus(by) ENT(abbrev. for the Ear, Nose and Throat Department of a hospital).
16. Humiliated, given low mark for all to hear (8)
DEGRADED : Homophone of(… for all to hear) “D-graded”(given a D grade/a low mark, in a test, say)
18. Staff welcoming a deposit from Kuala Lumpur? (7)
MALAYAN : MAN(to staff/to fill the positions in an organisation) containing(welcoming) [A + LAY(to deposit/to set down).
Defn: Descriptive of someone or something …, formerly the capital city of the States of Malaya, and now the capital city of Malaysia. Thus, the answer is something of an anachronism.
20. Setter’s second puzzle? It’s hell (6)
EREBUS : 2nd letter of(…’s second) “Setter” + REBUS(a puzzle in which words are represented by pictures and letters).
Defn: …/the darkness below the earth/the underworld.
21. Susie’s taking a little nap (6)
SIESTA : Hidden in(… a little) “Susie’s taking“.
23. Notes on origin of recession from the Sun (5)
SOLAR : SO,LA(syllables representing two notes in the sol-fa musical scale) placed above(on, in a down clue) 1st letter of(origin of) “recession“.
A nice diversion today that elicited several smiles. NOBLE GAS & SPEED were my favourites. I hadn’t heard of the British cricketer from the nineeenth century, but somehow had heard of Boots. I keep forgetting that in the UK for some reason OR are “men”, even though I’ve encountered it a few times. I thought that 8d’s clue was a bit loose. I wasn’t aware of “ta-ra”; in the Antipodes we’re more likely to use “ooroo”. “hooroo” or perhaps “ta-ta”. I vaguely remembered ISINGLASS from somewhere but didn’t know what it was.
Lots to like about this . Thanks to setter and blogger. I needed the latter for parsing of 1d. I got the anagram but could not see how“he” fitted in. Oh dear.
Amazed that W G Grace is not well known in cricket- loving nations. He twice captained England against Australia.
SM, perhaps I’m not your average Australian!
Pleasant warm-down after Brendan in the Guardian, but even so NESTLE and STOP PRESS held out until I had all the crossers at the end and I took a while to see the B ED for ‘teacher’ at 13a.
I particularly liked the sporting references. I don’t remember having seen Sonny LISTON in a crossword before, ‘Boxer’ usually being the man who defeated him, Cassius Clay as he was then. No problems with (Dr. W. G) GRACE; we have at least a couple of books about him and a Spy print of him hanging in the hallway. Glad to say that he would have had nothing to do with today’s designer stubble fashion!
Thanks to Moo and scchua
I am old enough to remember when we pronounced 1A as “nessels” and also, when I started working with a food manufacturer, we started correctly to say “ness-lay”.
I solved 3D without really understanding it. I took “heart of stone” to be the centre of “stone” giving “ton” as the last three letters’ but I couldn’t work out how “lis” meant champion. Thanks scchua for the explanation.
Funny that we had “OPPO” yesterday as a “mate” and today (14A) as a rival.
This was such a relief for me after yesterdays debacle with GUY. in 19d, capitalizing Speaker was a great misdirection. Thanks Moo and scchua, who I haven’t seen for a while – love his explanations!
Thanks for the blog , I thought the clues were very near and clever today. Will just pick out NOBLE GAS for the use of He and SOMBRERO for Top Gear.
Peter @5 , Nestle were partly to blame for “nessels” . In the 70s ” nessels Milky Bar ” was part of an advertising jingle which ran for several years.
I quite agree about Scchua’a explanations and photos. They are great fun. Thank you
All solved in the end, though we were held up in the NW corner by confidently bunging in ‘mules’ for 9ac until 2dn just had to be STOP PRESS. But LOI was CEDE, a real PDM when we got it. GRIMACE, SOMBRERO amd NOBLE GAS (once we got 9ac right) were our favourites.
Thanks, Moo and scchua.