STEERPIKE provides this morning's puzzle…
Some lovely surfaces here, a fair bit of tricksy parsing, and thoroughly enjoyable throughout.
Thanks STEERPIKE!

ACROSS
1. American ship left Spain following essentially pointless skirmish (6)
TUSSLE
(US (American) + S (ship) + L (left) + E (Spain)) following [poin]T[less] (essentially)
4. Philosophical treatise written by graduate about society (8)
ABSTRACT
TRACT (treatise) written by ((BA)< (graduate, <about) + S (society))
9. A former partner is said to be appropriate (5)
ANNEX
"an ex" (a former partner, "is said go be")
10. Expert tricks extremely uncivil govenor (9)
PROCONSUL
PRO (expert) + CONS (tricks) + U[ncivi]L (extremely)
11. Trendy joint starts to earn reputation on street (7)
HIPSTER
HIP (joint) + (E[arn] R[e[utation] (startts to) on ST (street))
12. Gifted crime boss dined with Democrat (7)
DONATED
DON (crime boss) + ATE (dined) with D (democrat)
13. Mellow river that is crossing Portugal (4)
RIPE
R (river) + (IE (that is) crossing P (Portugal))
14. Defending regime in TV broadcast is extremely harmful (8)
VIRULENT
defending RULE (regime), (IN TV)* (*broadcast)
17. Lesson about Scandinavian revolutionary protecting East German dissident (8)
RENEGADE
RE (lesson) about ((DANE)< (Scandinavian, <revolutionary) protecting (E (east) + G (German)))
19. Resent two letters, reportedly (4)
ENVY
"n v" (two letters, "reportedly")
22. Stall inexperienced reporter, caught in elaborate lie (7)
CUBICLE
CUB (inexperienced reporter) + (C (caught) in (LIE)* (*elaborate))
24. Steerpike’s favourite just discovered motivation (7)
IMPETUS
IM (Steerpike's) + PET (favourite) + [j]US[t] (discovered)
25. Improvised reggae set is unexpected bonus (6,3)
EASTER EGG
(REGGAE SET)* (*improvised)
26. Remove tackle from river in college grounds, primarily (5)
UNRIG
R (river) in UNI (college) + G[rounds] (primarily)
27. Learn from cultural reference involving old retired nobleman (8)
MEMORISE
MEME (cultural reference) involving (O (old) + (SIR)< (nobleman, <retired))
28. Prepare to run twisted experiment, say (3,3)
GET SET
(TEST (experiment) + EG (say))< (<twisted)
DOWN
1. Crate mostly containing top of range whisky (8)
TEACHERS
TEA CHES[t] (crate, mostly) containing R[ange] (top of)
2. Bird from Norway captivated by melancholy musician (9)
SANDPIPER
N (Norway) captivated by (SAD (melancholy) + PIPER (musician))
3. Unknown factor in non-clerical types’ negligence (6)
LAXITY
X (unknown factor) in LAITY (non-clerical types)
5. Family died partaking in winter sport? Horrific! (13)
BLOODCURDLING
BLOOD (family) + (D (died) partaking in CURLING (winter sport))
6. Oddly, Tory measure is best (7)
TROUNCE
T[o]R[y] (oddly) + OUNCE (measure)
7. A tragic heroine upheld virtue (5)
ASSET
A + (TESS)< (tragic heroine, <upheld)
Tess of the d'Urbervilles, presumably
8. Commotion engulfing large English city (6)
TOLEDO
TODO (commotion) engulfing (L (large) + E (English))
10. Coin collectors cruelly pestering Mark (7-6)
PARKING METERS
(PESTERING MARK)* (*cruelly)
15. Blend of ten rustic flavours (9)
TINCTURES
(TEN RUSTIC)* (*blend of)
16. Vision certainly embraced by rowing team (8)
EYESIGHT
YES (certainly) embraced by EIGHT (rowing team)
18. New uniform meets with unambiguous sort of reaction (7)
NUCLEAR
N (new) + U (uniform) meets with CLEAR (unambiguous)
20. High-pitched sound of note held in Beat It (6)
SCREAM
E (note) held in SCRAM (beat it)
21. Penny gets in pickle for partner (6)
SPOUSE
P (penny) gets in SOUSE (pickle)
23. Bust crown of skull in explosion (5)
BOSOM
S[kull] (crown of) held in BOOM (explosion)
Not a typical Monday puzzle but an enjoyable one nonetheless.
5d was my favourite. Needed Teacow’s explanation for VIRULENT and to put me right with SCREAM where I had thought of ‘beating cream’ and rather lazily, entered the answer without checking.
Needed all the crossers to spot 1d’s ‘whisky’, my LOI.
Thanks for the 1a, Steerpike.
Not an easy start to the week with some difficult parsing. I eventually managed to work out most of it but couldn’t see how RENEGADE worked, not having seen RE for ‘lesson’ before. I also had the wrong def (‘Philosophical treatise’) and therefore part of speech for 4a.
Some good defs in ‘best’ misleadingly as a verb at 6d and the ‘Coin collectors’ at 10d. Like Diane @1, my favourite was BLOODCURDLING.
Thanks to Steerpike and Teacow
Steady progress – tougher than usual for a Monday, I thought. Needed a bit of help from Thesaurus and thank you for explaining the Re in Renegade, which I’ve never seen as “lesson” before. There are some marvellous surfaces and precise, if misdirecting, cluing.
Parking Meters, Bloodcurdling and Annex, were perhaps favourites
Well done all, and thank you.
Thanks Steerpike and Teacow
17ac: RE (religious education) is probably worth remembering as a school subject. Less common variations on this are RI (instruction) and RS (studies).
Neat stuff
PB @4, thanks for explaining how ‘lesson’ = RE; I should have recognised it, but will hopefully remember it for next time.
Thanks Steerpike. I echo Teacow by saying that this was “thoroughly enjoyable” and I was struck by the abundance of smooth surfaces. I found this easier than other offerings by this setter and I had a slew of favourites including IMPETUS, EASTER EGG (neat anagram), GET SET, SANDPIPER, ASSET, EYESIGHT, NUCLEAR, and SCREAM. Thanks Teacow for the blog.
Reading Riting and Rithmetic are the Rs that I was aware of but didn’t know about RS. Have to watch for that. Thrown by S = ship by why not . Heigh ho
Thanks for the blog, very tidy set of clues, Tony@7 has my favourites in his list.
Another good challenging puzzle. I liked everybody else’s favourites.
Surprised to find RE (& its predecessor RI) had caused trouble for solvers here. I’m sure it was a stock crossword gimmick when I first started solving, but that was over 50 years ago. RE is still a compulsory subject for UK schools.
In 1a USS is just the American equivalent of HMS, so you don’t have to parse US & S separately. Does Chambers have S = Ship on its own?
In 20d Beat It appears (initially capitalised and in italics). Are we ignoring this pop culture reference?
Also I can’t see what the word “Defending” is doing in 14a.
I think “defending” is an inclusion indicator. RULE is inside the anagram of INTV .
Hi all, many thanks to Teacow for his generous review and to everyone else who has taken the trouble to comment. I think with FrankieG@10 and Roz@11’s comments, everything is parsed as I intended.
11 Roz@11:
Ah yes, I see. That wasn’t clear to me from the blog.
Thanks Steerpike & Teacow
Too late to comment, but I had to acknowledge the shout-out to my favourite sport at 5d BLOODCURDLING. Curling is a non-contact sport, so there is not much blood or death involved, but some skips are notorious for urging on their sweepers with 5 20s.
Thanks to Steerpike and Teacow for the fun.