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ACROSS

 1 In the distance, an expedition in Africa without any clothes (4)

 4 Passenger lists fine mess aboard parts of ship (9)

10 Person with property consisting of narrow passageway—mostly, it’s
  depressing (9)

11 Part of barrel? It’s ignited again (5)

12 Arnold, e.g., turned over bread with skill (7)

13 Professor’s funny bone (7)

14 Knitting equipment, demonstrating craft without necessity (10)

16 Doubtful second after the first (4)

18 Beginners in basketball exercised two hours one March (4)

19 “Packaging” sounds like psychiatrist talk (6-4)

24 Leader of radicals, in coalition, came together (7)

25 Exaggerated motion from fluid, wild cat (7)

26 On their faces, often, adult snakes exhibit slimy green spots (5)

27 Pests hiding in Western lair—don’t worry about it (5,4)

28 Filthy scum beginning to lose backward competition in sports coupe (6,3)

29 Flower reminiscent of Abner Doubleday, ultimately (4)

DOWN

 2 Strict believers give money—truly and surprisingly, it lasts (15)

 3 Better retool, if it helps keep you dry (4,4)

 4 Oft-repeated words from staff: “variable star” (7)

 5 Beast galloped north and west with Shakespearean prince (7)

 6 Discussion venues for you and me to hold symposium’s conclusion (6)

 7 Sell fifteen rare letters, reordered like this clue (4-11)

 8 Please speak about shaking fist (7)

 9 Too much Buddhism with a shaved head—that’s something many people
  can’t tolerate nowadays (6)

15 Chess ending with one rook and knight (3)

17 Test about rising musical genre in Tombstone locale (2,6)

18 Where lawyers meet for a drink? (7)

20 Frenzied din with echo of pleasure (7)

21 Dope clinches a victory over Republican pro (2,5)

22 Does up ridiculous phony (6)

23 Passes the Spanish fuel (6)

SOLUTION TO PUZZLE NO. 3373

ACROSS 1 FO(RCE)PS (ecr[u] rev.) 6 anag. 11 PO(UNC)E 12 init. letter 13 S(T)PAT (taps rev.) 14 HOP + E 15 [e]MAN + TRAP (rev.) 16 pun 18 POT(T)ERS (presto anag.) 20 P(HON)ING 22 “pear” 23 [s]-PACE-S 25 W + ARP 26 P[a]INTER 27 anag. 28 S(PIC + I)LY/span>

DOWN 2 [l]OATH[e] 3 R(N)A 4 alternate letter 5 rev. 7 S + LATE 8 “coat” 9 hidden 14 HEN + RY[e] 17 ANAL[ysts] 18 [f]OR + T.S. 19 REAL[m] 21 O + WE 23 anag. 24 2 defs.