Another pleasant solve from Doc …
… but marred by another mistake. The rubric said: the unclued lights can be arranged to form an unbroken chain with the last three letters of one word becoming the first three of the next one. There were 9 unclued lights but in order to make the chain work one needed 30d, which was clued. It took me a long time trying to join 17 to 20 without it … maybe the Spectator should search under the floorboards for gremlins.
The chain (which of course can begin anywhere) goes thus: 15d PONCHO; 33a CHOSEN; 31d SENSEI; 31d SEICHE; 3d CHESTY; 21d STYLUS; 6a LUSTRE; 17d TREMOR; 30d MORTAR; 20d TARPON
Words I had to look up: SENSEI – a martial arts teacher (Japanese); SEICHE – fluctuation from side to side of an enclosed body of water; TARPON – a large fish related to the herring.
The title, of course, while suggesting mailing that gathers momentum, literally reflects the recurring, joining letters.

Across
1 Play mind games with some gypsy children (5)
PSYCH
Hidden (some) in gyPSY CHildren
4 Skimpy petticoat found at fielding position (5,4)
SHORT SLIP
Skimpy petticoat = short slip
9 Series of clues including note on opponents, browbeaten (10)
THREATENED
THREAD (series of clues) around (including) TE (note) + N and E (opponents at bridge)
11 Sporting Cup seen in Ulster city, when cycling (5)
RYDER
DERRY (Ulster city) cycled around. The Ryder Cup is contested by the US and Europe at golf
12 All we Brits have a say in it (7)
ENGLISH
Cryptic definition: all (or most) Brits speak (have a say) in English – though not according to Professor Higgins …
14 Custom banger lacking appeal upfront (5)
USAGE
SAUSAGE (banger) without (lacking) SA (sexual appeal) in front
15 Small dog eats nothing for tea (5)
PEKOE
PEKE (small dog) around (eats) O (nothing)
21 Drunk, one ruins family do’s (8)
REUNIONS
Anagram (drunk) of ONE RUINS
22 Old coin in empty Eastbourne train (7)
EDUCATE
DUCAT (old coin) in E + E – Eastborne without content (empty)
24 Late Chopin piece: I Loved Narcissus (4)
ECHO
Hidden in latE CHOpin. Echo was a sea-nymph who fell in love with Narcissus
25 Stepped back to sulk in Stirling (4)
DORT
TROD (stepped) backwards. Definition – to sulk in Scottish
27 Society friend embraces that girl who’s perfect in form (7)
SPHERAL
Society = S + HER (that girl) inside (embraces) PAL (friend)
28 Eeyore’s friend twice to express his disgust (4-4)
POOH-POOH
Pooh Bear: twice – Eeyore’s friend in A A Milne stories
34 Horse circling western tree (5)
ROWAN
ROAN (horse) around (circling) W (western)
35 Farm vehicle backed up a kilometre on train line (5)
TRACK
TRAC (CART reversed – backed up) + K (kilometre)
37 Disturbed vigil in home, regularly, of clan from Angus (7)
OGILVIE
Anagram (disturbed) of VIGIL + O and E (home, regularly). The Ogilvies have been an Angus clan since 1143; Sir Angus was married to Princess Alexandra
38 NY state city once near Carthage (5)
UTICA
Double definition
39 Read-outs on rebuilt blue property (6,4)
EUSTON ROAD
Anagram (rebuilt) of READ-OUTS ON. Euston Road is a property in blue in Monopoly
40 Much water for vitamin supplements provider (5,4)
SEVEN SEAS
Double definition – Seven Seas is a commercial firm supplying vitamin supplements
41 Sign of tooth decay not initially conquered (5)
ARIES
CARIES (tooth decay) missing C (conquered initially)
Down
1 Crimson cloth on ancient ruler shows largest British butterfly (6,7)
PURPLE EMPEROR
Double definition: PURPLE is the term for the crimson-dyed covering of Roman emperors, and the Purple Emperor is a butterfly – though not the largest in Britain. Maybe largest is serving as synonym for Emperor, though Chambers doesn’t offer this.
5 Such landscaping’s a laugh (2-2)
HA-HA
Double definition: an invisible sunken ditch and a laugh
6 Welsh feature in Green EU review (7)
RENEGUE
Anagram (review) of Green EU. To “welsh” is to back out of something – now, I believe, non PC
7 Tennis star views around centre of Wimbledon (5)
SELES
SEES (views) around L (centre of WimbLedon). Monica Seles reached the final of Wimbledon in 1992
8 It will keep you up at night (8)
INSOMNIA
Cryptic definition
10 Book actors missing parking around hairdresser’s (13)
THESSALONIANS
THESIANS (thespians without P – parking) around SALON (hairdressers)
13 Doc uses new and old currency (7)
ESCUDOS
Anagram (new) of DOC USES. Definition: former currency of Portugal
18 Some hermitage is harbouring hostess (6)
GEISHA
Hidden in hermitaGE IS HArbouring
19 It’s so rude lifting jumper in error (7)
BOORISH
Reverse (lifting) of ROO (jumper) in BISH (error)
23 Double-talk! (8)
DUOLOGUE
Cryptic definition
26 Fliers for short volume with non-U letter (7)
TOMTITS
TOM(e) – tome – volume -without its end (short) + TIT(u)S – letter from St Paul without U
29 Wild crocus is found (6)
OCCURS
Anagram (wild) of CROCUS
30 Building material going a bomb (6)
MORTAR
Double definition. This entry should not have been clued
32 Spenser’s greeting to Stan Laurel, off and on (5)
SALUE
Odd letters (off and on) of StAn LaUrEl (Spenserian form of salute)
36 Frozen princess turned over just the same (4)
ANNA
Princess in Frozen reading the same when turned over
First‑time commenters will receive a verification email. Once verified, your comments will be auto‑approved.