The Glady Marsh by Salamanca

This was an intriguing puzzle from Salamanca. The unclued 1ac, 1dn gave WRITING ON THE WALL, referring to the theory of 5dn GREGSON and 17dn LESTRADE. In the Sherlock Holmes story, A Study in Scarlet, there was a clue in the word RACHE written on the wall where the body was found. Gregson and Lestrade thought it was the name RACHEL which had not been completed.

The answers to the asterisked clues were all names of girls missing their last letters. The full title could be amended suitably to be THEA GLADYS MARSHA.

Of course, in the story Gregson and Lestrade were wrong and Holmes had the correct interpretation. -"One other thing, Lestrade," he added, turning round at the door: "`Rache,' is the German for `revenge;' so don't lose your time looking for Miss Rachel."

With which Parthian shot he walked away, leaving the two rivals open-mouthed behind him.

Across
1ac, 1dn See preamble (16, four words) WRITING ON THE WALL
10* Coin, one with wings on back (5)
ANGEL 2 definitions
12 Spaceman Dan's challenges (5)
DARES 2 definitions ref. Dan Dare
13* Cascade pool beginning to dry (4)
LIND LIN + D(ry)
14 Become "A Study in Scarlet" (6)
REDDEN cryptic definition RED DEN (study)
15 Speaker of seditious words causing panic in Greek animals (12)
LEASING-MAKER Anagram GREEK ANIMALS
17* God's central to nun (3)
LAR (c)LAR(e)
19 Money (capital) and a weight (3)
MNA 2 definitions + M (money) + ‘N (and) + A
20* What was given to Cerberus at opening to Hades (4)
SOPH SOP + H(ades)
23 Fellow, an old knave, coming back for a hiding (7)
COVER-UP COVE + PUR (reversed)
24 Terrain is uneven: does this make it _______? (7)
SIERRAN Compound anagram TERRAIN IS less IT
26* Name is continental (4)
NEST N (name) + EST (IS in French)
27 In search of love in France ... (3)
FOR O (love) in FR (France)
29 ... some can discover this as well (3)
AND Hidden
30 Big Bear wants to go dancing as a groover? (12)
RABBETING-SAW Anagram BIG BEAR WANTS
34 Possibly stacked matter against one (6)
RECKAN RECK (to matter) + AN
36 Cook ragout: this may be a vital ingredient (4)
OKRA Hidden
37 Duplicate note covering one time and time again (5)
DITTO I (one) + T (time) + T in DO
38 Article replacing old clumsy stone container (5)
STEAN Anagram STONE replacing O (old) to A (article)
39 Sleep under church ground thus? (12)
ENSEPULCHRED Anagram SLEEP UNDER + CH; & Lit.
 
Down
1 See 1ac WALL
2 One's head was filled with sound intelligence primarily? Exactly the reverse (6)
IGNARO O(ne) + RANG + I Reversed
3 Street yobs: some created style (4)
TEDS Hidden
4 Lack of protein-developer. Fate? (5)
NORNA  No RNA
5 See preamble (7) GREGSON
6 Not even what addict might do with source of drugs? (3)
ODD OD (overdose) + D(rugs)
7 Long walk taking time about reaching its conclusion (4)
TREK T (time) + RE (about + (wal)K
8 Hemp-producer's even found in hard slate (not English) (8)
HENEQUEN ENE (even) in H (hard) QU(e)EN
9 Denuded land by cutting down trees wantonly on mass of soil (8)
ESTREPED Anagram TREES + PED
11 Remained killed after being beheaded (4)
LAIN (s)LAIN
16 Make recommendation to bow once in play and go (4)
MOVE 3 definitions
17 See preamble (8) LESTRADE
18 Colouring-matter recipe that's popular one Indian district made up originally (8)
ALIZARIN A ZILA (reversed) + IN (popular)
19 For myself, I start to rage, getting stuck in this? (4)
MIRE I + R(age) in ME
21* Catherine's veg recipe (4)
PEAR PEA + R (recipe) Ref. Catherine pear
22 Language of some Highlanders etc (4)
ERSE Hidden
23 Alternative technology fixing loop in copper cross (7)
CATTABU AT (alternative technology) + TAB (loop) in CU (copper)
25 One doing piecework who requires time to start off (6)
TASKER T (time) + ASKER
27 Decisive hand one's left after (5)
FINAL FIN (hand) + A L (left)
28 Ours rests here? (4)
ONUS ON US
31* Live telecasts, showing start and finish: they're laid on for horse-racing (4)
BETS BE (live) + T(elecast)S
32 Had hard living in Dacia? (4)
GOTH GOT (had) + H (hard)
33* Sorry dead stick (4)
WAND WAN + D (dead)
35 Hill in SA: one of Scotland's hill's round (3)
KOP KIP with O for I