Thank you to Shabbo. Definitions are underlined in the clues.
Across
1. Nice here in secretive island (6)
SICILY : ICI(“here” in the language of Nice, France) contained in(in) SLY(secretive/sneaky).
Defn: … in Italy.
4. Exciting to return 50% of wealth (8)
ELECTRIC : ELECT(to return a candidate in an election) + 1st 3 out of 6 letters of(50% of) “riches”(wealth/valuables).
10. Bridge odds on two queens (7)
SPANNER : SP(abbrev. for “starting prices”, indicating the odds of winning in a horse race) plus(on) [ ANNE(English queen once) + R(abbrev. for “Regina”/Queen) ](two queens).
Defn: A structure that bridges/spans across, say, a river, valley, gorge:
… or canal even: 
11. Dawn insures building (7)
SUNRISE : Anagram of(… building) INSURES.
12. Consumes escalope after turtle soup for starters (4)
EATS : 1st letters, respectively, of(… for starters) “escalope after turtle soup“.
13. Let bloke from Bow attach architectural feature (10)
IMPEDIMENT : ‘IM(“him”/that bloke/man, as spoken by one from Bow in the London East End) plus(attach) PEDIMENT(an architectural feature, viz. the triangular upper part of a classical building).
Defn: …/a hindrance.
15. Lower River Test cutting (6)
DEMOTE : DEE(a river in N Wales and NW England) containing(… cutting) MOT(in the UK, an annual test for motor vehicles’ roadworthiness, from the abbreviation for the Ministry of Transport).
16. Extends contract and steps down (7)
RESIGNS : Double defn: 1st: Signs another contract on expiry of the previous one; and 2nd: … from a post/job position.
20. Friends in bars discussed people behind bars (7)
INMATES : Homophone of(… discussed) “inn mates”(how one might describe friend/mates in various inns/bars).
Defn: …, prison bars, that is.
21. Green bananas the beginning of your vitality (6)
ENERGY : Anagram of(… bananas) GREEN + 1st letter of(the beginning of) “your“.
24. Top wine collection mentioned in book? (4,6)
BEST SELLER : BEST(top/unbeaten) + homophone of(… mentioned) “cellar”(s stock/collection of wine, often to be found in a, well, cellar).
Defn: Possibly/? a best-selling ….
26. Split taps work (4)
CHOP : CH(abbrev. for “cold” and “hot” respectively, as found on water taps) + OP(abbrev. for “opus”, a major musical or literary work).
28. Joy of endless intimacy (7)
ELATION : 1st and last letters deleted from(endless) “relations”(sexual or otherwise intimacy/intercourse).
29. Impact of Tory diplomacy (7)
CONTACT : CON(abbrev. for a Conservative Party member/a Tory) + TACT(diplomacy/discreetness).
Defn: …/action of coming together, forcibly)
30. Beach borders start of terribly unproductive land (3-5)
SET-ASIDE : SEASIDE(the beach/coastline) containing(borders) 1st letter of(start of) “terribly“.
Defn: …/land that is taken out of production to reduce crop surpluses.
31. Schumann’s opening surprisingly lofty for piano (6)
SOFTLY : 1st letter of(…’s opening) “Schumann” + anagram of(surprisingly) LOFTY.
Defn: …, the musical direction of play softly.
And, of course, Robert Schumann was a composer and pianist.
Down
1. Grounds to invest in South America (8)
SUSPENDS : SPEND(to invest/to employ, say, time or effort, doing something for a cause) contained in(in) [ S(abbrev. for “south”) + US(abbrev. for the United States of America) ].
Defn: Prevents an activity from happening, such as the social activities of child who is being punished.
2. Female replaces husband in conglomeration of merchants and artisans (9)
CRAFTSMEN : F(abbrev. for “female”) replacing(replaces … in) “h”(abbrev. for “husband” in) anagram of(conglomeration of) “merchants“.
3. Fast or slow having nothing to lose (4)
LENT : “lento”(musical direction to play slowly) minus(having … to lose) “o”(letter representing 0/nothing).
Defn: The … undertaken by Christians in the period before Easter.
5. Former magazine auditor (8)
LISTENER : Double defn: 1st: … published weekly by the BBC.
6. Demands trains (10)
CONDITIONS : Double defn: 1st: Requirements that must be met before something else can or be allowed to happen; and 2nd: To mould behaviour in a certain way, as Pavlov did with his dogs.
7. Suggest upgrade to rear lift (5)
RAISE : Multiple defns: 1st: …/bring ups, say, an issue for discussion; 2nd: …/to promote, say, from one class to another above; 3rd: …/to nurture babies or children to adulthood; and 4th: …/to physically or metaphorically bring from a lower to a higher position.
8. Reacts badly in icy run (6)
CRESTA : Anagram of(… badly) REACTS.
Defn: The act of travelling swiftly on a toboggan down a passage of compacted snow and ice.
9. It doesn’t pay to upset America endlessly (5)
CRIME : Anagram of(to upset) [ “America” minus its 1st and last letters(endlessly) ].
Defn: From the saying that tells you why not to commit a criminal act.
14. Cooks joints in vessels (10)
STEAMSHIPS : STEAMS(cooks food by heating it in steam) + HIPS(joints between the torso and legs in primates or hind legs in quadrupeds).
17. Tricky opponents on Tyneside? (5-4)
NORTH-EAST : NORTH,EAST(representing players in opposing 2-person teams in the card game of bridge, where tricks are won).
Defn: …, located in the North East region of England.
18. Rested, having marked out playing field earlier (8)
RECLINED : LINED(marked out with lines, obviously) placed after(… earlier) REC(informal term for a recreational ground/a playing field).
19. Understanding myth pays off (8)
SYMPATHY : Anagram of(… off) MYTH PAYS.
22. Kisses Boris bottling up torment (6)
OBSESS : Hidden in(… bottling) reversal of(… up, in a down clue) “Kisses Boris“.
Defn: To fill one’s mind continually with troubling/tormenting thoughts.
23. Remarkably cheap fruit (5)
PEACH : Anagram of(Remarkably) CHEAP.
25. Elegant form of transport on the rise (5)
SMART : Reversal of(… on the rise, in a down clue) TRAMS(a form of public transport, viz. vehicles powered by electricity from overhead cables and running on rails).
27. Cancel a French party (4)
UNDO : UN(French for “a”) + DO(a party/a social function).
Thanks, S and s!
Liked IMPEDIMENT, INMATES, RAISE and NORTH-EAST.
In SPANNER we have SP+ANNE+R.
Thanks KVa. Slip corrected.
Pleasant Sunday puzzle with the highlight for me the quad def RAISE. I didn’t know SET-ASIDE as a noun to describe land being taken out of production. RESIGNS is one of those interesting contronyms, even if the ‘Extends contract’ sense is usually spelt with a hyphen and is pronounced differently.
You probably meant this for 8d, but the CRESTA Run (‘icy run’) is a toboggan track in Switzerland which I seem to remember has strong British connections.
Thanks to Shabbo and scchua
I love Shabbo’s puzzles. His clueing is brief and accurate, and they always light and great fun to solve.
My repetition radar bleeped with the use of endless and endlessly to indicate removal of first and last letters, and I am not too keen on 30a, which I guess might qualify as a “EUism”! and, although it is in Collins, it is not given in Chambers.
I had a lot of ticks, with SPANNER, IMPEDIMENT, NORTH-EAST and, my favourite, RAISE making it onto my podium.
Many thanks to Shabbo and to scchua.
WordPlodder @3, thank you for introducing me to the term ‘contronym’. ‘Resigns’ often gives rise to much concern among football supporters when their favourite player is carelessly reported to have ‘resigned’! A very nice mental workout for a Sunday morning, satisfyingly taking exactly the length of breakfast time. Thanks Shabbo and Scchua.
A typical Shabbo puzzle, largely light but a lot of fun. Smiles across the grid but I’ll highlight SPANNER, IMPEDIMENT, LENT and RAISE.
Thanks indeed Shabbo and scchua
Well done, Shabbo, another very enjoyable solve.
Oops, forgot to add my thanks to scchua for the review!
Thanks, Shabbo & Scchua, this was very enjoyable fare for a Sunday morning.
I had examiner initially instead of listener, otherwise enjoyable Sunday solve. Thanks.
I would add RECLINED to the favourites others have mentioned. I. too, was expecting LISTENER to start with EX. I had BEST and TOP as verbs for BEST-SELLER, but I think it works either way. Thanks, both.
Not much to add to what’s already been said. SICILY and IMPEDIMENT were among our favourites.
Thanks, Shabbo and scchua.
Beaten up a bit by this one, being still fairly amateur at this.
RAISE was lovely, and chuckled at STEAMSHIPS.
Thanks to Shabbo and Scchua
I didn’t find this at all easy & eventually came up 1 short. Didn’t twig the wordplay for IMPEDIMENT, though should have done, & not familiar with let/hindrance so the definition escaped me. As ever with a Shabbo puzzle well clued & lots of fun. Plenty of ticks but RAISE would have to take top spot.
Thanks both.
Thank you to scchua for the blog and to everyone else for their very kind comments.
Season’s greetings to you all.