Lord Salisbury cared ... little for people. His aloofness was enhanced by shortsightedness so intense that he once failed to recognize a member of his own Cabinet, and once, his own butler. At the close of the Boer War, he picked up a signed photograph of King Edward and ... remarked, "Poor Buller, ... what a mess he made of it."
BARBARA TUCHMAN, THE PROUD TOWER
N. NEWPORT NEWS
(The notation: lowercase letters indicate deletions, parentheses mark containments, asterisks indicate anagrams.)
A. Denigrates halfhearted gatherings at the church in the fens
BEeS + MIR (CH) ES
B. Preceding a margin of victory
A + HEAD (as in horse racing)
C. King Henry's yeoman kept time by the queen's cadence
R + H + Y + T + HM (Y = yeoman is apparently a standard abbreviation)
D. Luxuriated like a king in retirement
B (AS + K) ED
E. Faraway from here, but didn't care? Just the opposite!
ABSENT + MINDED (antonyms of "here" and "didn't care")
Whether ABSENTMINDED is hyphenated apparently depends on one's dictionary. (Fortunately, that fact doesn't affect the puzzle.)
F. Testimonial that usually can't be checked out
REFERENCE: double definition (with reference to the reference section in a library)
G. A quiet German cardinal met the French missionary
A + P + OST + LE ("cardinal" as in direction, thus OST)
H. Pasted spread around ultimately with a sign of irritation
T (H + RASH) ED (last letter of "with")
I. Like a vestal virgin, entirely beyond comprehension?
UNKNOWABLE: double definition ("know" in the biblical sense)
J. Small function integrating productive factors
CO (EFFICIENT) S (cosine abbreviation)
K. Variety of English rustic breakfast?
HODGE + PODGE
While the word is simple enough, the clue ventured well into the obscure (although "English rustic" might be slightly better than "(Samuel) Johnson's cat"). A simple pun might have been better, as in "Rhyming salmagundi?"
L. Number one animated devil: it initially retreats behind the worldly
MATERIAL + IZATION (NO + I + TAZ + I reversal)
MATERIALISATION was accepted. Given that the usual non-US usage would be -ISATION (and -ISE: the Z in this answer appears in the word "recognize" in the quotation), and given a possibly unfamiliar reference (TAZ: the Tasmanian devil from the Looney Tunes cartoons -- that's a TASmanian devil), it wouldn't be fair to penalize (penalise?) solvers in this case. It might have been better to more explicitly denote the Z, as in "Appearance of a shattered Italian atomizer." Even then, perhaps some solvers might have written -ISATION from force of habit.
M. A financial official has antique coins, primarily silver and gold
ASSES + S + OR (plural of AS)
In the US, an assessor is a government official (often at the county level) who evaluates property for tax purposes. This may not have translated transoceanically.
N. Headlines follow the jazz festival in southeastern Virginia
NEWPORT + NEWS
O. Set aside a utensil not quite at maximum capacity
TABLE + SPOON + FULl
P. A scientifically trained snake charmer?
This doesn't quite work. Although "scientifically trained" and "snake" do (almost too simply) suggest HERPETOLOGIST, the attempt to misdirect isn't perfectly accurate. This might have been better if it were less specific, as in "One scientifically trained to work with scales?"
Q. Avoids flights when King Kong is cut
ESCHEWS: ESCAPES, APE becomes HEW
R. Sedative obliterated the initially brutal labor pain
PHENOBARBITAL: (THE B LABOR PAIN)*
Apparently this word isn't in the Chambers Dictionary.
S. Element restricted by a fellow
R + A + DON
T. Eccentric manner isn't rehearsed
OFF + HAND
U. Like a sheep set rolling round university in America
U (LOT + RICH + O + U) S
Another example of dubiety: ULOTRICHOUS, while describing woolly hair, isn't applied to sheep, only humans -- so "like a sheep" is at best an indirect definition.
V. The least sensible feasted riotously?
W. Service almost controlled by a cartel perhaps in southern India
TRI (CHINa) OPOLY (TRIOPOLY, while uncommon, is reasonably inferable.)
X. Perpendicular bit on a line primarily describing a motorcycle
ORT (HOG) ON + A + L
Y. Lincoln once hit back at mysterious contrivance
WHIG + MAL (LAM reversal) + EERIE (Abraham Lincoln was a member of the Whig Party when he served in Congress.)
Z. Gave directions to a party couple
E + N + DO + WED
a. Hideously stretched and prepared to be broken
RACKED (double definition, as in torture and pool)