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ACROSS

 1 Stop! Fold right away (5)

 4 Essential components of some engines: creamed corn (I jest) (9)

 9 Yokel put cows back, sheltering eight times as many (7)

10 Crazy, mixed-up aunt sits in lap, mostly (7)

11 Penny leads a demonstration downtown (9)

12 Prosecutor goes after precious car (5)

13 African animal is well ahead of a detective (5)

14 500 kids returning around current winter migrant (8)

17 Border kind of property of the stars (8)

19 Leaders of organized unions try delivering our best (5)

22 Parts of a church expunging foremost of errors (5)

24 Redo components, repeating some in a new sequence (9)

26 Embrace supernatural powers and love to do drugs (7)

27 True motivation for corrupt senator’s betrayal (7)

28 How one might be caught, holding no spades or clubs? (3-6)

29 Armstrong’s unusually clean weapon (5)

DOWN

 1 PDQ’s musical proficiency? (4-4)

 2 Frank rising up in support of worker’s organ (7)

 3 Windows release obscured by eeriest, arcane knowledge (9)

 4 Perfect passenger to ride along (5)

 5 Jam with rock musician Morton (5,4)

 6 8⅓-foot piece of cake? (5)

 7 Laundry additive is unopened more frequently (7)

 8 Friendly note involving spies (6)

14 Woman eating fruit with commercial pioneer (9)

15 Type of curve enveloping John Washington: it can be white and protective (5,4)

16 To make a long story short: stupid scam comes first (8)

18 Tasteless fool lives with Nickelback (7)

20 Activates unit around containers (5,2)

21 Vocation of horse’s ass: interrupting someone who gives a damn (6)

23 Diving from small island (5)

25 Chose large reference work about Barnum (5)

SOLUTION TO PUZZLE NO. 3340

ACROSS 8 F + I + SCAL[p] 9 anag. 10 hidden 11 WELLS + POKE + N 12 anag. 14 DISAR (anag.) + M 16 [arm/D]-ADA 18 hidden 19 [sed/H]-ATE 20 anag. 21 RAN + DOMLY (anag.) 23 C([p]OLL[y])ARBON + E 26 2 defs. 27 NA (rev.) + R-RATED 28 anag.

DOWN 1 “whizzed, um” 2 S(CRAP + HE)APS 3 [cal/F]-LOW 4 A(T A LOSS FOR WOR)DS (sorrows float anag.) 5 HOR(SEDRAW)N (was red anag.) 6 [dom/W]-INO 7 SEW + ERR + AT 13 SOL + IDAR(IT)Y (diary anag.) 15 SCHO(OLD + E)SK (shocks anag.) 17 ARI(ZO + NA)N (an oz. rev.) 22 anag. 24 [ten/L]-URE 25 [ner/E]-VES