Repeated thanks to those who submitted clues for this.
My opinions are below. The winner is revealed at the end.
Luciano
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Duke:
Big mouth from Mississippi, maybe, changing regarding recent killer? (5, 7)
Big mouth, etc. = DELTA; changing = VARIANT; def = recent killer
-- The two indicators for DELTA and VARIANT aren't that cryptic. The surface reading, I presume, alludes to vociferous doubters re the virus, its guises and its avoidance, but it isn't terribly elegant.
New problem emerging when interval data reclassified (5, 7)
INTERVALDATA anag; def = new problem
-- A very good find with 'interval data', which is/are a real thing, I find. The definition, while relevant now, is a bit vague generally.
Dangerous but ignored threat attired, naval-style (5, 7)
ATTIREDNAVAL anag; def = dangerous but ignored threat
-- Not a huge fan of the way the anagram is indicated. Not quite sure what to make of the surface reading. The definition, as with the previous clue, is a bit vague.
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Mark:
To many ducking the jab, a death blow is dealt? (5,7)
DEALT = DELTA 'VARIANT'; def = to many ducking the jab, a death blow
-- I'm not the hugest fan of using an implied anagram indicator as part of the definition, but I have seen it in many another clue. The reference to 'jab' gives the definition some specificity (cf Duke's previous two clues).
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David:
". . . or NOT to be" belated confusion everywhere it seems to this dreadful development (5,7)
BELATED anag minus BE = DELTA, with 'confusion' serving as anag indicator as well as pointer to VARIANT(?); as per David: "everywhere" alludes to Hamlet's "sea of troubles" and Coleridge's "water, water everywhere" seemingly!
-- I'm not sure how the allusions serve the clue or help the solver.
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My winner is Mark. Duke and his interval data is/are second.