I printed this puzzle within minutes after it appeared online and, like others, blinked with puzzlement that there were only two clues in the down column! Some creatively filled the gap with print-and-paste, or by copying longhand. I went to plan B, solving on the screen, which worked fine for me although some reported problems with that too.
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.