Great fun from Goliath.
A thoroughly enjoyable puzzle – simple but not easy, challenging but not hard.

ACROSS
1. Time to add salt (6)
SEASON
4. Spin monster’s advance (8)
PROGRESS
PR (spin, Public Relations) + OGRESS (monster)
9. Observe water or steam? (6)
NOTICE
10. Composed lyric half Gaelic in this script (8)
CYRILLIC
(LYRIC)* (*composed) + [gae]LIC (half)
12. One’s upset for a very long time (4)
EONS
13. Regularly quibble in what I’m doing now, providing a roof over someone’s head (10)
SUBLETTING
[q]U[i]B[b]L[e] (regularly) in SETTING (what I’m doing now)
15. Obtain silver for corrupt economic ends (4,2,6)
COME IN SECOND
(ECONOMIC ENDS)* (*corrupt)
18. Dental expert has a monthly event — it’s not changing (12)
PERIODONTIST
PERIOD (monthly event) + (IT’S NOT)* (*changing)
21. For Elton John, Kenneth became Hercules (6,4)
MIDDLE NAME
Cryptic definition (although not cryptic really…)
Elton Hercules John was born Reginald Kenneth Dwight
22. In Somalia, an African country (4)
MALI
24. Denial of actual bottom-pinching (8)
REBUTTAL
REAL (actual); BUTT (bottom) pinching
25. Tumble that hurts when not active (6)
FALLOW
FALL (tumble) + OW (that hurts)
26. Look back with wine for a memento (8)
KEEPSAKE
PEEK< (look, <back) + SAKE (wine, Japanese)
27. Absolutely not taking over right to be a nation (6)
NORWAY
NO WAY (absolutely not) taking over R (right)
DOWN
1. Time for a few words (8)
SENTENCE
2. Crazy amount you endlessly wasted to get independence (8)
AUTONOMY
(AMOUNT YO[u] (endlessly))* (*wasted)
3. Previously a Spanish team? (4)
ONCE
Double definition
The ONCE team were a highly successful Spanish cycling team, actually sponsored by ONCE. Click here for more.
5. Prince, perhaps a rogue, gets half- hearted TV award in art venue (5,7)
ROYAL ACADEMY
ROYAL (prince, perhaps) + A + CAD (rogue) + EM[m]Y (TV award, half-hearted)
6. Bright to be paying attention after midnight (10)
GLISTENING
LISTENING (paying attention) after [ni]G[ht] (mid)
7. Pastry served up in memorial ceremony (6)
ECLAIR
[memo]RIAL CE[remony]< (served <up in)
8. Feudal rent saturation reported (6)
SOCAGE
“SOAKAGE” (saturation, “reported”)
11. Meson, it somehow splits another subatomic particle? (8,4)
QUESTION MARK
(MESON IT)* (*somehow) splits QUARK (another subatomic particle)
14. Crash diet lapses with bread and butter here (4,6)
SIDE PLATES
16. Bar from which Goliath would come back looking unhealthy (8)
DISALLOW
I’D< (Goliath would, <come back) + SALLOW (looking unhealthy)
17. Steps leaving street for a breather (8)
STAIRWAY
[st]AIRWAY (a breather, leaving ST (street))
19. National Rail’s trademark designed with no red elements (6)
AMTRAK
T[r]A[de]MARK* (*designed, with no RED elements)
From United States
20. Fit to eat by consumer on vacation? That would be plausible (6)
EDIBLE
C[onsume]R (on vacation) + EDIBLE would be plausible
23. Previous leader’s love for county sauce (4)
MAYO
MAY (previous leader, Theresa May) + O (love)
With two straight definitions
Loved this. Eminently solvable, but witty throughout, with no obscurities or over-contrivances.
Question mark is a classic among a rich crop.
Thanks Goliath and Oriel for the blog.
I enjoyed this too.
23d was clever with two definitions.
Thanks Goliath and Oriel. First class from both.
In 3d I took ONCE as just the Spanish word for eleven (a football team), rather than the (obscure?) cycling team.
Didn’t know either meaning for the ONCE team and I always thought (wrongly, obviously) that SOCAGE was pronounced sockage rather than soakage, so I missed that one. But yes, great fun: I liked NOT ICE.
Tiny quibble: if there are to be no red elements in TRADEMARK, surely you have to remove both Rs?
Never heard of SOCAGE, which I had to reveal — is it a British thing? The Spanish team and Elton’s nomenclature were beyond me, and I couldn’t parse EDIBLE. Otherwise all good.