The puzzle wasn’t particularly hard, but featured lots of diversity. I thought it had many clever clues. So much so that it’s hard to name an exception. Many clues required thought, but the clue of the day has to be 13dn with its punning literary reference. I wrote it in but couldn’t parse it until I eventually appealed to my wife. Thanks to the setter for a very enjoyable puzzle.
Clues are in blue, with definitions underlined. Answers are in BOLD CAPS, then wordplay. (ABC*) means 'anagram of ABC'. Deletions are in [square brackets].
1 Want party to exchange leaders? That’ll bring hostile reaction! (8)
BACKLASH: swap leading letters of LACK BASH. Nice to be spared mention of Dr Spooner.
5 Volume four including books English offered as promised (6)
VOTIVE: V=volume, OT=a set of books commonly mentioned in crosswords, IV=four, E=English. I knew “votive” had religious significance, but not that it meant this.
8 One swoops near ground, moving left to right (3)
OWL: start with LOW=near ground, and follow the instructions!
9 Consent for each assignment (10)
PERMISSION: PER / MISSION. Geddit?
10 No longer confined, holding on for advocate? (8)
EXPONENT: EX PENT, holding ON.
11 Girl brought to mark that locates treasure chest (6)
THORAX: THORA is “Miss Saturday” this week. X marks the location. Make sure you separate “treasure” from “chest”.
12 Australia finally getting spinner on (4)
ATOP: add TOP to [Australi]A. The definition is almost invisible! And no, not a cricketing clue.
14 Came down after snooze, subdued (10)
RESTRAINED: RAINED after REST.
17 What one has to go on is fresh and amusing (5,5)
GREEN LIGHT: GREEN=fresh, LIGHT=amusing. Definition relates to traffic signals.
20 Authentic quartet to be seen in Liechtenstein (4)
ECHT: hidden answer.
23 Animal, better left in two areas, sent west (6)
ALPACA: LCAP=CAP / L[eft] “sent west”. Top and tail it with 2 x A=area.
24 Decline to accept drink, having answer ready for Bulgarian (8)
STOTINKA: SINK=decline, “accepting” TOT=drink, then A=answer. As it happened, I’d just seen the same answer in the previous week’s Jumbo with an almost identical type of clue, so this was a “gimme”. Definition is Bulgarian currency.
25 Motorway route cut short as data misread (10)
AUTOSTRADA: (ROUT- AS DATA*), “cut short” and “misread”.
26 Works informer shifting Republican to the centre (3)
ART: RAT with R=Republican moved as instructed.
27 Impulsive thing in north going after new money (6)
NEURON: N=new, EURO=money, N=north. Neutrons transmit nerve impulses.
28 Sit and adore rippling body (8)
ASTEROID: (SIT ADORE*) “rippling”.
1 Recoil from report about distraught Romeo (9)
BOOMERANG: BANG=report, around “distraught” (ROMEO*). Some boomerangs, not all, do return but I would dispute that they recoil. Alas, it’s in the dictionary!
2 Music from revolutionary agent welcomed to state ball (7)
CALYPSO: YPS=“revolutionary” SPY, in CAL O (State, ball).
3 Raised in family like some wild animal (6)
LUPINE: UP in LINE.
4 Tiger seen abroad where lion holds sway? (9)
SERENGETI: (TIGER SEEN*) “abroad”.
5 Six men accommodating model tourist (7)
VISITOR: SIT “accommodated” by VI=six, OR=other ranks.
6 Fossil — see black one with dull exterior (9)
TRILOBITE: LO=see, B=black, all inside TRITE=dull.
7 Excellent plonk given at raves (7)
VINTAGE: (GIVEN AT*) “raves”.
13 Rhymester's introduction to The Pit and the Pendulum say (9)
POETASTER: The Pit and the Pendulum is a short story written by Edgar Allan Poe, so an introduction to it might be a Poe “taster”.
15 Islanders and old volunteers whacked Scotsmen (9)
TAHITIANS: TA=old volunteers, HIT, IANS=(certain) Scotsmen.
16 Action needed about land taken away (9)
DETRACTED: DEED around TRACT.
18 Rare vacant tenancy agreement — it's free! (7)
RELEASE: R[ar]E is “vacant”, followed by LEASE.
19 Appears following trouble over affair (7)
LIAISON: LIA=AIL “over”, IS ON=appears (at the Alhambra, say).
21 Tax in one country initially raised money abroad (7)
CENTAVO: VAT in ONE, followed by C[ountry] … all “raised”, since this is a down clue. It’s a currency unit, mainly in South American countries.
22 Gunners captured by amphibious creatures mounting low attack (6)
STRAFE: RA in EFTS “mounting”.