WITH A GRAIN OF SALTARELLO
a 3×6 word rectangle
Noam D. Elkies, 12/2013
(annotated Feb.3, 2014 for Mendelssohn’s 205th birthday)

SOLUTIONS

1-Across can be either A_MINOR or A_MAJOR, because the 3-Down and 4-Down clues work for both:

1 2 3 4 5 6    ACROSS
A M I N O R    1   Key of Mendelssohn’s “Italian” Symphony
C E T E R A    2   The “c.” of “&c.”
K V E T C H    3   Make like an “oy!”ster
1 2 3 4 5 6    ACROSS
A M A J O R    1   Key of Mendelssohn’s “Italian” Symphony
C E T E R A    2   The “c.” of “&c.”
K V E T C H    3   Make like an “oy!”ster
DOWN

1  Half of a weapon vs. the Luftwaffe
2  An e−e+ annihilation releases at least 1.022 of these (abbr.)
3  Sulf- suffix
4  New York ballplayer
5  LotR’s Lugdush, for one
6  Half of a cheer

Across entries:
1A – Though often listed as being simply “in A major”, this symphony ends with an A-minor movement (an unusual choice, as briefly explained in the introductory paragraph of that Wikipedia page), so it can be regarded as being in either major or minor, and should be described as being just “in A”. This final movement’s title “Saltarello” is taken from a Neapolitan dance with a “peculiar leaping step”, whose name is cognate not with “salt” but with “somersault” (from the Latin verb “to jump”).
2A – &c. = et cetera (NB the ampersand “&” is originally a ligature for et).
3A – The “oy-ster” pun is not new but I haven’t seen it used in a crossword clue.
[as it happens each of the three Across clues — and none of the six Downs — uses “double quotes”.]

Down entries:
1D – “ack-ack” is not onomatopoeia but the pronunciation of “AA” in an early British spelling alphabet. AA stands for Anti-Aircraft (or perhaps Anti-aircraft Artillery), so a crossword clue for ACK via “ack-ack” should not mention “A.A.” or “antiaircraft” or “artillery” as one occasionally sees, including one clue for ACKACK itself.
2D – MeV = mega-electronvolt. e− and e+ are respectively electron (negatively charged) and positron (the electron’s positively-charged antiparticle); each has a rest mass whose E=mc2 equivalent in energy is about 511 keV = 0.511 MeV, so when they annihilate at least 0.511 + 0.511 = 1.022 is released (possibly more if they collided at a significant fraction of the speed of light). This is an important enough constant that most of the top Google hits for 1.022 annihilation show MeV.
3D – Sulfite and sulfate are the SO3 and SO4 dianions (or their salts). There are several other such pairs (nitrite/nitrate may be the best-known alternative); I chose sulf- for alliteration with “suffix”.
4D – Since I detest using such proper-name clues for perfectly good English words like JET and NET (each of which has at least two different cluing directions), the appearance of such a clue here is also a subsidiary hint at the multiple solutions. (As it happens the NYC area also has a ball team called the Mets, in case you ever need a J/M or M/N quantum square.) For that matter, if you thought 1-Across was simply A_MAJOR, why would I clue the plain word ATE as a chemical suffix?
5D – LotR = Tolkien’s epic Lord of the Rings trilogy. No, I’m not enough of a fanatic to come up with this clue off the top of my head… Fortunately Wikipedia’s List of Middle-earth Orcs included at least one that made this alliterative clue possible.
6D – As in “rah-rah”, and opportunistically complementing 1-Down with another “Half of a …” clue.