One doesn't really think of racehorses as heroes, but this was otherwise a well-constructed and fairly challenging nod to the Grand National. Given the abbreviations declared in the preamble , the superfluous wordplay letters spelled out the message "Omit L[letter] and mix up DCA [definition clue answers]. Omitted Ls[letters] spell K[keyword]. Mixes are K[keyword] four". Following these directions, the solver found that each definition-only answer could be anagrammed to a Grand National winner plus an extra letter. The extra letters spelled out "National" as the keyword.
The three unclued answers were HEROES, RUSSIAN and ERITREAN. HEROES could be similarly anagrammed to HORSE + E. The other two are clearly "Nationals" and Eritrean can be anagrammed to AINTREE + R, thus providing the thematic place name (and for Gregson, causing the thematic penny to drop). Finally, the answer CROWS OVER can be read as wordplay leading to the horse RED RUM ( = MURDER rev.), providing an amusing end to an entertaining puzzle, as well as explaining why perhaps the best known Grand National winner was not otherwise included.
Definition Only
Attentive
REGARDANT à N + RAG TRADE
Recurring pattern
TALEA à A + TEAL
Port in Kent
RAMSGATE à T + SEAGRAM
Field position (two words)
IN GOAL à I + ANGLO
Village in Kent
WROTHAM à O + MR WHAT
Having a good time
PARTYING à N + GAY TRIP
Mean
BASE à A + E.S.B.
Queue for handouts
BREADLINE à L + BINDAREE
Across
6 Work in particular district, then take rest (6)
S[o]PIDER (OP inside SIDE + R (take))
11 Mountain pasture a calm place? Not entirely (3)
AL[m]P (Hidden in cALM Place)
12 Checks quiet ones (6)
STUN[i]TS ('ST + UNITS)
13 Fair result (4)
EVEN[t] (Two meanings)
14 Game having turned Leeds almost coming back with points (5)
URDEE[l] (RU rev. + LEED(s) rev.)
16 Member of community immediately seized by fit leaving chapter
(8)
CANONESS[a] (ANON inside CESSA ??)
18 Is triumphant about consummation (be it quite short) (9, two
words)
CROW[n]S OVER (CROWN (consummation) + SO + VER(y))
20 Ate extremely horrid flies - a very small amount (4)
HAET[d] (Anag. ATE + H + D (first and last letters of HORRID))
21 Loved ones are interrupting cleaner who generally gets imposed upon (7,
two words)
JOE S[m]OAP (JOES (loved ones) + A(re) inside MOP)
23 Argue with second mate (4)
SPA[i]R (S(econd) + PAIR)
24 Excess reported before navy gatecrash (4)
[x]SORN (XS (homophone of EXCESS) + OR (before) + N(avy))
29 Doctor half ruined us - meant this substance to be injected
(9)
ANTISERUM[u] (Anag. RUI(ned) + US + MEANT)
32 Gasps about English stakes (5)
[p]ANTES (E(nglish) inside PANTS)
25 Sailor shortly began to be absorbed in book of rules (6)
POPE[d]YE (OPE'D inside PYE)
38 Girl is entertained by large number of Indians (4)
LISA[c] (IS inside LAC (large Indian number))
39 Establish wacky formula for Radio One (6)
ORDAI[a]N (Jumble RADIO + AN)
40 Crazy cattle (3)
N[o]UT (Two meanings)
41 Queens not involved in slur being circulated (6)
RANEE[m]S (NE (not) inside SMEAR, all rev.)
42 Wear out ex-political leader not starting truce (7)
T[i]REAGUE (TIRE + (H)AGUE (William H., erstwhile Conservative leader))
Down
1 Wife upset is repellent, alas (8)
WA[t]ESUCKS (W(ife) + ATE (upset) + SUCKS)
2 Highest region on heath, not motorway, giving unrestricted admission
(8)
[t]OPEN-DOOR (TOP + END + MOOR minus M(otorway))
3 Point elegantly expressed raised by government (4)
TA[e]NG (NEAT rev. + G(overnment))
4 See preamble (6)
HEROES à E + HORSE
5 American lectured on floor of theatre (4)
AREA[d] (A(merican) + READ)
6 Shame dictator (5)
STA[l]IN (Two meanings)
7 Seems lifestyle has divisive effect upon football team (8)
[s]PURPORTS (PORT inside SPURS)
8 Racing form wrong on day - gutted (4)
[s]INDY (SIN + DAY minus middle letter)
9 Domestic prisons are finally involved in revolution (4)
ESNE[p] (PENS + E (last letter of ARE) all rev.)
10 Move unsteadily round square falling over - try again (6)
RETE[e]ST (S(quare) inside TEETER rev.)
15 Boy lifted post (4)
[l]EWEN (NEWEL rev.)
16 Acceptable - that's surprising (3)
COO[l] (Two meanings)
17 Start of sketch with parrot: could be dead? (3)
S[k]EA (First letter of SKETCH + KEA (parrot))
19 Corporation lawyer (French) set up cellar (4)
VA[m]UT (TUM + AV (Avocat, French lawyer))
22 So lifeless, one family of fish (8)
S[i]CARIDAE (SIC + ARID + AE)
23 A number protecting nurse, young family member (4)
SIEN[x] (EN (Enrolled Nurse) inside SIX)
25 Sportsman helps to make shoe aromatic (3)
O[e]AR (Hidden in shOE ARomatic)
26 Steamer in function (3)
[s]SIN (SS (steam ship) + IN)
27 Figaro's role involves European language (6)
BE[a]RBER (E(uropean) inside BARBER (Figaro's profession))
28 I favour powdered derris (5)
SIDER[r] (Anag. DERRIS)
30 The main palisaded enclosure: wood (6)
S[e]APELE (SEA + PELE)
31 Angel endlessly going back to rising lawyer (5)
EB[k]LIS (EBB minus last letter + SILK rev.)
33 Craft teacher absent (4)
PRO[f]A (PROF + A(bsent))
34 Force adult deer to disappear (4)
FAD[o]E (F(orce) + A(dult) + DOE)
26 Blow on climbing plant (4)
ONE[u]R (ON + RUE rev.)
37 Pen name beginning to embarrass story teller (4)
ELIA[r] (First letter of EMBARRASS + LIAR)