It turned out to be a relatively straightforward exercise for a Saturday, except for my struggles with the NW corner. LOI was 9ac, immediately following 2dn. I enjoyed the gentle pun at 2dn, and was fascinated by the unfamiliar language at 25ac. Thanks to the setter for a very enjoyable puzzle.
Clues are blue, with definitions underlined. (ABC*) means ‘anagram of ABC’. Deletions are in [square brackets]. The blog is in Times New Roman font, as part of a gentle campaign to urge the club site to use a font in which it is easier to tell one’s stem from one’s stern.
1 Debussy's piece is less convincing (5)
LAMER – La Mer was composed by Debussy.
4 Angels, heading off to restrict devil, providing introductory thoughts? (9)
EPIGRAPHS – [s]ERAPHS restricting PIG.
9 Plant provided by gardening girl to cover ridged structure (9)
MACHINERY – MARY, as in Mary Mary quite contrary, ‘covering’ CHINE. ‘Chine’ may be a regional noun, but having visited Poole, I’d heard of Branksome Chine.
10 Live broadcast's core element fine (5)
DWELL – D is the central (‘core’) letter of ‘broaDcast’. WELL is ‘fine’.
11 Just a house — use as place of retreat (6)
HONEST – HO (house), NEST (place of retreat). On edit: thanks to special bitter for pointing out that if you're prepared to read a long dash as a minus sign, "house — use" says to drop the last three letters off to give the HO at the start of the answer. Very nice!
12 Half-hearted saint with inadequate bunch of fighters (8)
LUKEWARM – LUKE, W (with), ARM[y].
14 Dumped old females given confidential advice? (6,3)
TIPPED OFF – TIPPED (dumped), O, F, F.
16 Room for improvement? (5)
SALON – I think this is just a cryptic definition of a salon, as a place you go to improve your mind or appearance.
17 Cold, something inclined to give one pain (5)
CRAMP – C (cold), RAMP.
19 Some maybe in seats support opponents at match (9)
BACKSIDES – BACK (support), SIDES.
21 City, finally beaten 'ollow, wailed (8)
YAMMERED – [cit]Y, [h]AMMERED.
22 Good old man embracing modern music? That's the spirit! (6)
GRAPPA – G (good), PA, ‘embracing’ RAP.
25 Language of fierce female putting son off repeatedly (5)
TIGRE – TIGRE[ss], dropping two sons. It’s apparently a Semitic language of northern Eritrea and adjacent parts of Sudan.
26 Dead, then ending in Elysium? It's different for this lot! (3,6)
THE DAMNED – (DEAD THEN M*). A delightful &lit. definition.
27 Idiot gets tied up outside and watched (9)
MONITORED – MOORED outside NIT.
28 Cox maybe in vessel that's lost in the morning (5)
STEER – STE[am]ER is the vessel. To cox is one way to steer.
1 My, my, the plastic's disrupted biological network! (9,6)
LYMPHATIC SYSTEM – (MY MY THE PLASTICS*).
2 On a dry day no one will want this wine (5)
MÂCON – On a dry day, you don’t need a MAC ON.
3 Concerned with matter for discussion being put in print again (7)
REISSUE – RE, ISSUE (matter for discussion).
4 Upset some oversexed old lovers (4)
EXES – backwards hidden answer (‘upset’, ‘some’)
5 Aggressive or nice, somehow bottling a fury that's silly, about nothing? (2-4-4)
IN-YOUR-FACE – (NICE A FURY*) ‘about’ an O. There are two anagram indicators, ‘somehow’ and ‘silly’, but an anagram inside another anagram (‘bottled’), is basically just a big anagram.
6 Soldiers needing ceremonial attire correct (7)
REDRESS – RE, DRESS.
7 Quiet minister, excellent, went ahead and won the day (9)
PREVAILED – P, REV, A.1., LED.
8 Marooned son's ill, abandoned individual in a Pacific location (7,8)
SOLOMON ISLANDER – (MAROONED SONS ILL*).
13 Sight in the heavens turns sailor into ferocious person (6,4)
DOUBLE STAR – the sailor is a TAR. If one DOUBLES TAR, one gets TARTAR, the ferocious person in question.
15 Tramp again disturbed shoos off a bird (9)
PTARMIGAN – (TR-MP AGAIN*). The A in ‘tramp’ is ‘shooed off’.
18 Offer from head of state, spending tiny bit of money as before (7)
PRESENT – PRES[id]ENT (head of state), with 1D (one old penny) ‘spent’.
20 Funny folk, the best sort during extremes of seriousness (7)
SCREAMS – CREAM inside S[eriousnes]S.
23 Box in church for money (5)
PENCE – PEN (box in), C.E. (church).
24 Crowd gathered in auditorium (4)
HERD – sounds like ‘heard’ (gathered) in an auditorium.