Hi everybody, and away we go …
The preamble:
Some across answers must be entered in a thematic way to emulate a path DOWNHILL. Wordplay in each clue generates an extra letter not required for the answer; in clue order, these spell out an instruction affecting 27 cells. Chambers Dictionary (2016) is recommended, but does not contain one entry which can be found in Collins English Dictionary.
I twigged the entry method early – as soon as NEANDERTHAL and TIKI clashed with ADRY and RETILE. My next answer, MR CHAD, confirmed it. It was soon apparent that it is the acrosses in every other line which are entered backwards (although the two palindromes BEEB and OTTO could go either way). So we are slaloming or otherwise threading our way DOWNHILL zigzag fashion.
Appropriately, this was a very top down solve for me. Although in a few places I was using the message to help me solve the clues, it appeared steadily enough. First, HIGHLIGHT T-R … IONS WITH STR …, then a bit later I knew I would be HIGHLIGHTing THREE somethings WITH STRAIGHT LINES. Finally:
HIGHLIGHT THREE SKI LOCATIONS WITH STRAIGHT LINES
It was all DOWNHILL from there. I hunted in the grid (including the main diagonals) for likely looking places, going downwards of course. I drew a blank. I looked up lists of ski resorts online. I gave in and asked a friend for a hint. And there they are. Brr! After that endgame I’m in need of warming up, so I’m off for some après ski …
Thank you Vagans for the cool puzzle!

| # | ANSWER | Clue with definition underlined | |
| Explanation, with quoted indicators in italics and ANSWER letters in bold caps | |||
| Across | |||
| 1a | CATACUMBAL | Fancy chum at cabal offering underground wine cellar (10) | H |
| An anagram of (fancy) C[h]UM AT CABAL | |||
| 11a | NEANDERTAL | Alarmed Nina alerted old man (10) | I |
| An anagram of (alarmed) N[i]NA ALERTED | |||
| 12a | TRULLO | Turkey and seabird beside old Apulian hut (6) | G |
| TR (Turkey) and [g]ULL (seabird) beside O (old) | |||
| 13a | TRACTS | Areas having temperature charts in disarray (6) | H |
| T (temperature) + C[h]ARTS anagrammed (in disarray) | |||
| 14a | THREEPIT | Scots insisted on it with nobleman held back by grip (8) | L |
| ’T (it) + PEER (nobleman) reversed inside (held back by) HI[l]T (grip) | |||
| 15a | TIKI | Dent missing from identikit returning image of ancestor? (4) | I |
| DENT is missing from the reversal of (… returning) [i][dent]IKIT | |||
| 16a | OMEN | Sign showing honorific information (4) | G |
| OM (honorific) + [g]EN (information) | |||
| 17a | RAISABLE | Frazzled hair and fur apt to be brought to life (8) | H |
| An anagram of (frazzled) [h]AIR + SABLE (fur) | |||
| 20a | IDAEAN | Vagans has appointment with a nationalist from mountain in Crete (6) | T |
| I (Vagans) + DA[t]E (appointment) + A + N (nationalist) | |||
| 22a | BEEB | Broadcaster reversed prediction after openers of batting excelled (4) | T |
| Reversed BE[t] (prediction) after the first letters of (openers of) Batting Excelled | |||
| 23a | HASSARS | Catfish mince smells according to Alastair (7) | H |
| HAS[h] (mince) + SARS (smells according to Alastair) | |||
| 27a | OTTO | Bit of comfort to Bismarck? (4) | R |
| A bit of comfO[r]T TO | |||
| 29a | INTOMB | In old days bury book in wine case with front at back (6) | E |
| TOM[e] (book) in BIN (wine case) with its first letter, B, at the end (front at back) | |||
| 30a | NOAH’S ARK | Toy boat made from Korean ash (8, two words) | E |
| An anagram made from KOR[e]AN ASH | |||
| 31a | ARDS | Used regularly, man’s crudest primitive ploughs (4) | S |
| Used regularly, mAn’[s] cRuDeSt | |||
| 33a | NILL | Deactivate surrounding nuclear discarded refuse (4) | K |
| [k]ILL (deactivate) surrounding N (nuclear) | |||
| 35a | WEAR THIN | Organ boxed in behind the scenes to become less convincing (8, two words) | I |
| EAR (organ boxed) in W[i]THIN (behind the scenes) | |||
| 37a | ALLOTS | Parcels out everything: a great deal (6) | L |
| AL[l] (everything) + LOTS (a great deal) | |||
| 38a | GILGAI | Depression in Perth: reformed Congregationalism rejecting “wicked” monstrance (6) | O |
| An anagram of (reformed) [con]G[re]G[at]I[o][n]ALI[sm] removing (rejecting) an anagram of (wicked) MONSTRANCE | |||
| 39a | GALIONGEES | Turkish sailors look back after local girl with American child heading for Genoa (10) | C |
| SEE (look) reversed (back) after GAL (local girl), [c]ION (American child) and the first letter of (heading for) Genoa | |||
| 40a | CANNON | Drunk nana taken in by trick gun (6) | A |
| An anagram of (drunk) NAN[a] taken in by CON (trick) | |||
| 41a | KATI | Chinese measure of biscuit taken from the east (4) | T |
| Part of biscuIT [t]AKen backwards (from the east) | |||
| Down | |||
| 1d | COTTONS ON | Abstract notion Scot understands (9, two words) | I |
| An anagram of (abstract) NOT[i]ON SCOT | |||
| 2d | AHRIMAN | Amir Noah destroyed evil spirit in Persia (7) | O |
| AMIR N[o]AH anagrammed (destroyed) | |||
| 3d | TAUPE | Grey-brown bark thoroughly English (5) | N |
| TA[n] (bark) + UP (thoroughly) + E (English) | |||
| 4d | CALENDARS | Provide air staff in limos, maybe, where lists of dates are kept (9) | S |
| LEND (provide) A[s] (air staff) in CARS (limos, maybe) | |||
| 5d | MR CHAD | Wot no man protesting march about War Department (6, two words) | W |
| MARCH anagrammed (about) + [w]D (War Department) | |||
| 6d | BETTINA | Girl from Italy to remain with singer during afternoon (7) | I |
| BE (to remain) + T[i]T (singer) + IN (during) + A (afternoon) | |||
| 7d | ADRY | Poet’s needing drink that dreamboat dryad provides (4) | T |
| DreamboA[t] DRYad holds (provides) the answer | |||
| 8d | BACKBEAT | Secondary stress in bar behind hard black old food (8) | H |
| BACK (behind) + [h]B (hard black) + EAT (old food) | |||
| 9d | RETILE | Perversely sterile surface with slabs once more (6) | S |
| Anagrammed (perversely) [s]TERILE | |||
| 10d | ONST | Child welcoming new style once, some say (4) | T |
| [t]OT (child) taking in (welcoming) NS (new style) | |||
| 18d | SARMATIAN | Tsar with marina surprisingly at home on the Volga (9) | R |
| An anagram of (… surprisingly) TSA[r] with MARINA | |||
| 19d | EBLIS | Old Nick aged badly lying in sun at first (5) | A |
| [a]E. (aged) + initial letters of (… at first) Badly Lying In Sun | |||
| 21d | ENTAILED | Inevitably brought on a German train, European director (8) | I |
| E[i]N (a, German) + TAIL (train) + E (European) + D (director) | |||
| 24d | SWANSON | Final appearance for Gloria? (7) | G |
| SWANSON[g] (final appearance) | |||
| 25d | SERICIN | That woman with poison that is found in raw silk (7) | H |
| S[h]E (that woman) with RICIN (poison) | |||
| 26d | UNDEALT | Undivided melody has depth in it and high tone (7) | T |
| [t]UNE (melody) has D (depth) in it followed by ALT. (high tone) | |||
| 28d | TOLLED | Showed the way after signal demanded charge (6) | L |
| [l]ED (showed the way) after TOLL (signal) | |||
| 32d | RAGGA | Type of rap show held by most of Aussie air-force (5) | I |
| G[i]G (show) inside (held by) RAAf (Aussie air-force) without the last letter (most of …) | |||
| 33d | LASE | A lens transformed work as source of light (4) | N |
| A LE[n]S anagrammed (transformed) | |||
| 34d | ETNA | Early English worker mounted vessel (4) | E |
| E[e] (Early English) + ANT (worker) upwards (mounted) | |||
| 36d | WILI | Maiden’s spirit is under Wisconsin lake (4) | S |
| I[s] under WI (Wisconsin) and L (lake) | |||
There’s a Setter’s Blog for this puzzle here: https://bigdave44.com/2024/10/24/ev-1664-solution/
Thanks Kitty – the slaloming nature of the across clues had completely passed me by! Very neat.
One other small thing that struck me about the location names was that they were parallel. I’m no skier but I do know that you are supposed to keep your skis parallel.
Many thanks to Vagans.