A puzzle by eXternal … but not Clue Two IV (yet).
Preamble: Single letter clashes occur in six cells and each should be resolved in favour of a third letter to make crossing entries thematic. These letters can be arranged to form a word which, along with thematic entries, gives a location. In the final grid, an entry related to the location should be appropriately changed, also making crossing entries thematic.
I always have misgivings about “appropriately” in the preamble, as is commonly causes delay in the endgame. I didn’t solve more than a couple of across clues on the first pass (more success with the downs – nearly half), but I did find one clash: SHILLS and BELS in the top row, towards the right, to be resovled as SHELLS and EELS. Quite a lot of the top right quadrant fell into place – no more clashes – and then I made some headway with the lower half of the puzzle. 
The second clash appeared with THRASH and BALD in the bottom row, towards the left, to be resovled with either an E or a U, and placed symmetrically with the first clash; and then I saw FREIT and TRIAL in the leftmost column, near the bottom – no symmetry there – resolving with a U. The clash between CLAW and MARGOT (low, towards the right) brough to mind Margot Asquith’s correcting Jean Harlow’s mispronunciation of her first name: “No, no; the ‘t’ is silent, as in ‘Harlot’.”
Dug out the last 2 clashes, each with unambiguous resolutions, and wrote down those together with the other 4: MESUUM or MESUEM; it had to be the former and the location was MUSEUM, with the thematic entries being MARROW, MILLIPED, FRUIT, URIAL, PANSY, ROBINS, SHELLS, EELS, THRUSH, BALU, MARMOT, CLAM. So, that would indicate the (a?) Natural History Museum. But was that the location, or was the location the Cromwell Road, in SW7 (or elsewhere)? 
As I suspected, the “entry related to the location [that] should be appropriately changed” eluded me. The next day (or maybe be the one after that) I typed “Natural History Museum change” into a search engine of choice and immediately saw “dinosaur”, prompting me to remember diplodocus, or ‘Dippy the dinosaur’ which was replaced by … a blue whale. And effecting that replacement gives us REDWING, H, COAITA, LOACH, and SNIPE.
Thanks eXternal – but not one of your best for me, owing to my inability to rumble the endgame in an appropriate amount of time.
| Across | |||
|---|---|---|---|
| No. | Clue | Answer | Wordplay |
| 1 | Swallowtail, say, on black cart (6) | BARROW | ARROW (swallowtail) after B(lack) |
| 5 | Keen to lose Rex and sons, dodgy US sidekicks (6) | SHILLS | SHRILL (keen) − R(ex) S(ons) |
| 9 | Force used by chimp on enemy (6) | IMPONE | (ch)IMP ON E(nemy) |
| 11 | Read about new fieldwork (5) | REDAN | [READ]* N(ew) |
| 12 | Book about small vessel reaching island city (7) | LISBURN | LIB (book) around S(mall) URN (vessel) |
| 13 | Mistreated Chile ambassador (5) | ELCHI | [CHILE]* |
| 16 | About to secure tin with a catch (7) | INSNARE | IN RE (about) around SN (tin) A |
| 18 | Bats silly ignoring rook (5) | DIPPY | DRIPPY (silly) − R(ook) |
| 20 | Slap characters protecting interior of property (5) | PANDY | P AND Y (outer characters of “property”) |
| 23 | A band backing Prince abroad (4) | AMIR | A RIM< (band) |
| 25 | End of work in Harvard Institute, filling in two records (6) | EPILOG | I(nstitute) between EP (record) and LOG (record) |
| 26 | Insect’s form seen after changing sides in place in office (6) | INSTAR | INSTAL (place in office) with R(ight) for L(eft) |
| 27 | Make rotation to magnify twofold (4) | DUAL | LAUD< (magnify) |
| 28 | One of three sisters is gross one (5) | GRACE | GR(oss) ACE (one) |
| 30 | Leader of dippers, weary kind scratching wings (5) | FAGIN | FAG (weary) (k)IN(d) |
| 31 | Scotsman’s to snatch contents of yacht club slyly (7) | CLAUCHT | [(y)ACHT CLU(b)]* |
| 37 | Suffering of community after bishop leaves (5) | TRIAL | TRIBAL (of community) − B(ishop) |
| 38 | Dead flying creatures, reportedly insectivores (7) | NUMBATS | homophone: NUMB (dead) BATS (flying creatures) |
| 39 | Island abandoned by leader in hostile fashion (5) | ICILY | (S)ICILY (island) |
| 40 | Woman to work in trading place (6) | MARGOT | GO (work) in MART (trading place) |
| 41 | Move legs in swimming party (6) | THRASH | double definition |
| 42 | So far nets catch adult fish (6) | ANABAS | AS (so far) around NAB (catch) A(dult) |
| Down | |||
| No. | Clue | Answer | Wordplay |
| 1 | Ordered nurses to kiss, making you stimulated (8) | FILLIPED | FILED (ordered) around LIP (kiss) |
| 2 | Oliver, perhaps, removing top for mistress (4) | AMIE | (J)AMIE (Oliver, perhaps) |
| 3 | Belt holding in old boy’s apparel (6) | ROBING | RING (belt) around OB (old boy) |
| 4 | Charge extra when book’s released (4) | ONUS | BONUS (extra) − B(ook) |
| 5 | This narcotic Rose and I developed could make us millionairesses (10) | SINSEMILLA | [MILLIONAIRESSES − ROSE & I]* |
| 6 | Trees famous person is climbing (4) | BELS | SLEB (famous person) |
| 7 | Notes hair when bottom’s raised (4) | LAHS | LASH (hair) with last letter moved earlier |
| 8 | Tailor welly ultimately to be like a wader (5) | SNIPY | SNIP (tailor) (well)Y |
| 10 | Greek letters academician removed from recess (4) | PSIS | APSIS (recess) − A(cademician) |
| 11 | Scots separating in fight with sound of bell after signal to stop (7) | REDDING | DING (sound of bell) after RED (signal to stop) |
| 14 | To consume vermouth, get a sherry glass (6) | COPITA | IT (vermouth) in COP (get) A |
| 15 | Wrangle on a slope, caught in middle of hawthorn plant (10) | TRAGACANTH | RAG (wrangle) A CANT (slope) in (haw)TH(orn) |
| 17 | Leather approved before being used up (5) | NAPPA | APP(roved) AN (before) all rev. |
| 19 | Cook to encroach on land to hunt (5) | POACH | double definition |
| 21 | Make arrangement for ailing antelope (6) | NILGAI | [AILING]* |
| 22 | Composed guy only rarely behaving like a child (7) | YOUNGLY | [GUY ONLY]* |
| 24 | Checks in advance, demos where nothing should be source of error (8) | PRETESTS | PROTESTS (demos) with O (nothing) replaced by E(rror) |
| 29 | Clean underwear for British composer (6) | RUBBRA | RUB (clean) BRA (underwear) |
| 30 | Female on stage upended is omen in Macbeth (5) | FREIT | F(emale) TIER< (stage) |
| 32 | Cry from bird clutching large crab (4) | CLAW | CAW (ry from bird) around L(arge) |
| 33 | Principal supporter (4) | ARCH | double definition |
| 34 | Poor guards left without locks (4) | BALD | BAD (poor) around L(eft) |
| 35 | Car firm needing an agent in America (4) | G-MAN | GM (car firm) AN |
| 36 | Tailless beast covered structure of ancient times (4) | STOA | STOA(t) (beast) |
|
|
|||
A fairly quick solve here, and for once I spotted what needed to be done for the endgame without too much ado. An enjoyable diversion over a bitterly cold, snowy weekend.
I enjoyed this, particularly because I *didn’t* need to google for the theme. A remarkable number of natural history related answers squeezed into the grid. Glad I managed to find someone to scan me in a copy, given that no newspapers managed to make it through to us last week.
I’m with H___G____ here – not my favourite eXternal ever.
I don’t really like word searches at the best of times but when I have no idea what I’m looking for, it’s a gazillion times worse.
I had vague recollections of a dinosaur being moved but I thought that it was, most likely a T-Rex. I’m not sure if I’ve ever been to The Natural History Museum despite living so close to it at one stage. I have been in the foyer of The American Museum of Natural History in New York though it was too close to closing time to make it worth paying the entrance fee.
I finished the puzzle eventually but I didn’t have a WHALE of a time.
Thanks to S-er and B-er.
I had a similar experience to HG and Kenmac. I also wasted a lot of time trying to find the location in the grid – I knew it was the Natural History Museum – and was rather disappointed at the final denouement. I expected to have to find something hidden in the grid, not for it to be the answer to one of the clues. Perhaps I should have read the rubric more critically. Good grid fill, disappointing end game, but clever to be able to replace Dippy with whale and produce more living things!
Similar to the above really. A relatively straightforward grid fill (aside from the delay due to entering REM for the band at 23a) and then a lengthy bout of headscratching and trying to engineer specific location information from the thematic words. The NHM was the one to come to mind but I wasn’t totally convinced that the word plus the thematics gave a specific location. As HG notes in his blog (thanks), they could relate to any natural history museum?
Thanks to eXternal.
I was expecting a tough one after the last couple of weeks, and so it proved (for me, anyway). I really struggled with the top right corner (sleb?), and too unhesitatingly assumed Bale and Thresh were wanted, as they seemed somewhat on theme. I must admit, when I couldn’t see an obvious word connecting to a museum location, I gave up, so I can’t complain of wasting time. I rather assumed a museum had changed its entrance from one street to another… But I enjoyed the challenge.
Thanks to eXternal and HolyGhost
A DNF for me. I am fascinated how often I fail to finish puzzles that others find easy; happily it sometimes works the other way around.
I filled most of the grid but made a couple of mistakes by mis-spelling sensimila and entering INSTAL instead of INSTAR. I had not heard of dippy the dinosaur and knew nothing of his replacement so I doubt if finishing the grid would have helped me anyway.
Thanks to all.
I really enjoyed it
Same comments as Jon@1 except we solved this on a rather dull grey day in Lanzarote – at least it was warmer than the UK.
Thanks to eXternal and HolyGhost.