27874 Thursday, 14 January 2021 Unimpeachable

I found this rather easy and whizzed through in 11.31 scarcely pausing for breath and (truthfully) making sure the wordplay worked. Unusually, there is no “hidden” answer, and no every-other-letter clue, and only two anagrams to entertain you. For the longer one, I freely admit I got the answer before checking all the ingredients were there: they are. I’d be interested to know if anyone worked it the other way round.
The Greek valley might raise a few eyebrows, but it’s been here before, and I was a bit uncertain of spelling needed for the South American country and the African city.
The editor/setter must be congratulated for slipping in 14d: I spent much of last evening watching events unfold (again) in the Capitol, in rather more orderly fashion.

Below is my discussion of the clues, showing definitions and SOLUTIONS

Across
1 Family name of Swiss banker taken in by scam (8)
COGNOMEN Originally “the last of the three names of an ancient Roman indicating the house or family to which he belonged; a surname; an epithet or nickname.” So now you know: a familiar word now with a specific meaning. Formed here by CON for scam taking in a GNOME, conventionally of Zurich, and precisely a Swiss banker. So not a river, for once.
5 Lizard in S American region, one moving to prime position (6)
IGUANA The South American region is (French) GUIANA (hence the spelling) home to Devils Island. Move the 1 (I) to the front, the prime position.
10 Achieve prominent publicity making banners? (3,3,9)
HIT THE HEADLINES I sort of get this, since banners are newspaperspeak for HEADLINES, but I’m not entirely sure where the HIT THE comes from. Making? Perhaps it’s just a cryptic definition.
11 Illustrious celebrity entertaining the Spanish Left (7)
STELLAR The celebrity is a STAR, entertaining s in giving space to the (in) Spanish EL and L(eft)
12 Affair of big cat crossing road with last of cubs (7)
LIAISON As a choral singer, I can’t hear liaison without thinking it should have a kyrie in front, but here the big cat is a LION, the road it crosses is the A1 with the last (letter) of cubS providing the rest.
13 Confusion brought about by inspector’s instruction (8)
DISORDER The (Detective) Inspector’s instruction is the DI’S ORDER
15 Liberal leaving place of worship in wooded vale (5)
TEMPE A place I know only from this one, but it’s in Chambers as the valley of the Peneus in Thessaly, praised by the ancient poets for its unsurpassed beauty. It’s in heaps of poetry as a place of outstanding beauty. Here’s Catullus:
From Tempe’s vale next ancient Peneus came,
That fertile vale immortalized in fame!
Where Messos’ blue-eyed nymphs delight to rove,
Tempe o’erhung with many a circling grove!
To get your own economy sized Tempe, take the L(iberal) out of the TEMPLE place of worship.
18 Wizardry of fighting man in rainproof garment (5)
MAGIC Raincoat MAC (what else) and fighting man therein GI
20 Neatly trim thick hair covering current dog (8)
MANICURE My last in’ fooled into thinking of haircuts. Thick hair is MANE and the current dog is I (symbol for electric current) and CUR
23 Taking notice of ambassador, say, limiting noise (7)
HEEDING Ambassadors to the court of crossword are HE (His/er Excellency) and say is EG, the two surrounding DIN for noise
25 Gibbon, for one, a senior cleric (7)
PRIMATE The well known identity of apes and bishops
26 Great hopes Berg cultivated about a stage work (3,7,5)
THE BEGGARS OPERA Our first anagram (cultivated), of GREAT HOPES BERG and A. It would have been brilliant if the music for the 20th century version (The Threepenny Opera) had been Alban Berg’s, but it was Kurt Weill’s.
27 Money you and I brought back for evergreen tree (6)
CASHEW Hands up if you knew the source of the yummy nuts was an evergreen. Money is CASH, and you and I backwards produce the EW
28 Cheek of learner leaving lucky creature outside hotel (8)
BACKCHAT Quite cute. The lucky creature is a BLACK CAT, take out the L(earner) and add elsewhere the H(otel)

Down
1 Joint party-giver’s charge for keeping house (6)
COHOST Charge id COST and HOuse is kept therein. Chambers prefers it with a hyphen, as do I, otherwise it’s salmon with a T on the end.
2 Understanding crowd (9)
GATHERING A very simple double definition
3 Eponymous hero’s extremely offbeat greeting (7)
OTHELLO Is Othello a hero? Desdemona had cause to disagree if not for very long. Anyway, extremely OffbeaT plus HELLO for greeting gives the man and his play
4 Woman finally changes sides, finding compound (5)
ETHER Start with your random woman ETHEL and change her L(eft) to R(ight)
6 Good worker everyone held to be brave and chivalrous (7)
GALLANT G(ood) plus ALL for everyone and ANT for the “worker”, as so often.
7 Woman’s name concealed by mature son (5)
AGNES Should be followed by Dei (see 12 above). N(ame) is “concealed” by AGE for mature in verbal form, and tagged by S(on)
8 No-show sailors put on course right away (8)
ABSENTEE Sailors are ABS (Able Bodied) and the course from which you remove the R(ight) is ENTRÉE. I toyed with a D on the end, thinking “put on course” was ENTERED, but it doesn’t work.
9 Witch briefly keeping old Scottish pot for boiling (8)
CAULDRON The witch is a CRONE, cut short as indicated, and she “keeps” AULD for the Scottish version of old.
14 US politician eager to crush Yankee rabble-rousing (8)
DEMAGOGY The surface is undoubtedly today the redoubtable Nancy Pelosi successfully charting the House through Impeachment 2.0. The less exciting crossword version is DEM for American politician (other makes are available) eager: AGOG on top of (NATO) Y(ankee)
16 African city reached by libertine in thirty-one days (9)
MARRAKECH At least one other spelling available, trust the wordplay. MARCH is one exemplar of 31 days, and libertine supplies RAKE therein.
17 Forceful chap I met at sea (8)
EMPHATIC Only our second anagram (at sea), of CHAP I MET
19 Egyptian citizen displays anger, breaking stick (7)
CAIRENE Another slightly unlikely spelling, so again trust the wordplay, anger: IRE inside stick: CANE
21 Outstanding feature primarily of go-ahead Native American (7)
CHINOOK Outstanding feature (in most people) is CHIN. Promarily Of gives the first O, and go-ahead in the sense of giving it provides the OK.
22 Vehicle has got hired to go round old Mediterranean area (6)
LEVANT Formerly the eastern end of the Med. VAN the vehicle has LET the hired round it.
24 English archdeacon’s betting odds (5)
EVENS An archdeacon is styled VEN(erable). Put E(nglish) on the front.