3989: Key: Cutting by Lavatch

This was a complex puzzle by Lavatch which Gregson found very satisfying. The letters omitted from across clues gave THE RAPE OF THE. In Down clues the first letters of the extra words could be rearranged to form COMBINATION. This word and the phrase given by the latent letters can both be followed by the word LOCK.

In the grid  could be seen the phrase TRIVIAL THINGS from the first lines of the poem.

WHAT dire Offence from am'rous Causes springs,
What mighty Contests rise from trivial Things,

Then, treating the whole grid as a combination lock, the columns had to be moved until the name ALEXANDER POPE replaced the words TRIVIAL THINGS.The three ambiguities were the repetition of the same letters in 3 columns.

When these had been resolved, the dénouement appeared as the phrase –
GEM-BELINDA-INI – a reference to the last lines of the Rape of the Lock.

This Lock, the Muse shall consecrate to Fame,
And mid'st the Stars inscribe Belinda's Name!

Gregson cut out the columns and moved them up and down until the messages were apparent. Fortunately, it was not necessary to submit the altered grid!
 
Letters Latent are shown in square brackets.

ACROSS
1 German plot to capture a British crew (6)
GABBED G + BED containing A + B
6 Noble Frenchman widening prince's borders (7)
PALA[T]INE ALAIN in P(rinc)E
11 Acting part, one gathered inspiration may be drawn from this (6)
AIR[H]OLE I in A (acting) + ROLE
12 Scandinavian city's old and stupid (6)
ODENSE O (old) + DENSE
13 Specific religious instruction essential: time needs setting aside (7)
TRIVIAL VITAL + RI moving T
15 Tennis player dropping fifth of service points (5)
HING[E]S HINGIS less I (5th letter of SERVICE)
17 English are arrogant, millions wanting to confirm (6)
[R]EASSU[R]E E (English) + ASSUME less M
18 A number wrongly assigned to work on fortification (8)
TEN[A]ILLON TEN + ILL + ON
20 After mixture is safe, ion undergoes hydrolysis (9)
SA[P]ONIFIES anagram IS SAFE ION
23 Austria's people invaded by leader of Nazis vote for this? (9)
ANN[E]XATION N(azis) and X (vote) in A NATION; & lit.
29 Note idiot getting shot before the end? (8)
AP[O]STILLE STILL (photo) in APE
31 Estate's sheep dog losing weight (6, two words)
[F]EE TAIL EWE less W (wight) + TAIL (to dog)
33 Writer beginning to stir pulses (5)
DAHLS DAHL (ref Roald D.) + S
34 With pistol, huntsman's shielding Eastern republic (7)
DAGES[T]AN E in DAG (pistol) + SAN
35 River bordered by city showing growth (6)
LIPOMA PO in LIMA
36 Guarding the border (6)
[H]EDGING 2 definitions
37 Seen this rank for marine's lead? (7)
ADMIRAL Compound anagram MARINE’S LEAD less SEEN; & lit.
38 Saintess tortured, ie, lacking things to put out the flames (6)
SNAST[E]S anagram SAINTESS less IE

DOWN
2,25 Pilots destroyed marine [nuclear] institute: when packing shot, it's easy to manage (6;6)
AIRMEN Anagram
PLIANT I in PLANT
3,19 Musician deferring alto's appearance in hit electronic number [called] Black Moon Flower in which is unknown soprano expressing nature's disintegration (8, two words;8)
BRIAN ENO BRAIN with A (alto) moved + E + NO
BIOLYSIS B (black) + IO + LIS containing Y (unknown) + S (soprano)
4,24 Man with fluid is source of drinks [in] Georgia following long Russian river (6;6)
BOVRIL BO + VRIL
PINEGA PINE (long) + GA (Georgia)
5,32 In Glasgow, proceeds to chatter about base [Anglican] clergyman -- "a bit of demonic spawn" (4;4)
GAES E (base) in GAS
DEAN D (bit of demonic) + EAN (does spawn= give birth to?)
6,30 Footy games with lover of Scotland flipping [nervous]? One unfortunately suffering game finishing early, one-nil (5;5)
PEDAL PE (games) + LAD (reversed)
POLIO POL(O) + I - O
7,28 Famous Hungarian King astride horse [might] force soldiers to produce some armour (5;5)
LEHAR H (here horse=heroin) in LEAR
VISOR VIS + OR
8,27 In sea that's turbulent, plant [infolds] bits of crustaceans and sound of water's rising on island (5;5)
ANISE Anagram IN SEA
PALPI PLAP (reversed) + I
9,26 Drink in Earth covered by sunshine when climbing [through] wide, rolling dale and wooded country (5;5)
NEGUS GE in SUN (all reversed)
WEALD W + anagram DALE
10,13 Director beheaded many a real character? Writer [obviously] figures high death rate's about right (10;10)
EISENSTEIN (R)EIS (plural of REAL, the coin) + EN + STEIN (ref. Gertrude Stein?)
TETRAHEDRA Anagram DEATH RATE + R
14,21 Autocrat in Number 10 is leading [Britain] -- he's hardly to be believed in any way (5;5, two words)
TENNO TEN + NO (number)
AT ALL A TALL (unbelievable, as in ‘a tall story’)
16,22 Personal friend's thrown away money [on] "une femme affreusement sale" (4;4)
INTI INTIMATE less MATE
ELSA Anagram SALE (affreusement=frightfully)