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ACROSS

 1 Mountain habitat (zone I) restored, turning Athos into a host? (15)

 9 Frequent customer and returning nut seen in back (7)

10 Pavement’s acidity limited by a chemical compound (7)

11 Mathematical figure is 100 + 1 (4)

12 Sunny visual involving fog east of Hawaii (10)

15 Downturn almost perturbed embassy staffers (9)

18 Laid-back, being cured by pet (4,1)

19 Aircraft from Boeing originally lacking in strength (5)

20 Curls done a special way for a villain (9)

21 When it’s time for a coffee break, stand awkwardly with those who wait (10)

23 Leaders of ancient Moab observed some guy in the Old Testament (4)

26 Whaler beheaded fish near West London, at first (7)

27 What P might say: “Paint” (a lot of it) (7)

28 23 flipped in his presence, smashing a kitchen appliance (8,7)

DOWN

 1 Alakazam! One undergarment on either side of 20 (11)

 2 Heathen exalted North American retail chain (5)

 3 Rights organization: “Don’t finish reading what you’re reading” (4)

 4 Redesign a purse—one for Czechs, say (9)

 5 Arab’s nest egg, supported by energy (5)

 6 Naming piece of software “Nothing in Color,” largely (10)

 7 Coming up, Indian queen grabs ring and puts out (2,1,6)

 8 and 26 See water or steam (6)

13 Ear on venerated person is something usually replaced by 10, nowadays (11)

14 Spies infiltrating battle for pay (10)

16 Mysterious hints breaking up support for FedEx Office, perhaps (5,4)

17 Newly set forth slim one-reeler (5,4)

22 Maps without end, finally (5)

24 African uncle brought up a branch of computer science (5)

25 Barkeep: “Ice pick requires this repeatedly—it’s a long story” (4)

26 See 8

SOLUTION TO PUZZLE NO. 3343
 

ACROSS 1 PUR(LOIN)E + D +  LETTER 9 L(IPR)EAD (Rip anag.) 10 anag. 11 ANTONY[m] 12 LOOP + HO + LE (rev.) 14 CAB + LEG + RAMS 16 S + CAR 19 BEE + P 20 anag. 22 R + I(BOSOM)E 25 AU + BURN 28 anag. 29 W(HIT)ISH 30 KNITTED + ONE’S BROW (bores now anag.)

DOWN 1 POLK + A 2 anag. 3 O LEANDER 4 rev. hidden 5 hidden 6 anag. 7 TAN + GO 8 R(I + SEE)ARLY  (Larry anag.) 13 F(R)EE 14 CYBE(R + PUN)K (Becky anag.) 15 rev. 17 anag. 18 SE(QUOI)AS 21 ESCOR (anag.) + T 23 hidden 24 rev. (&lit.) 26 MO(H)ON rotated 27 SWA (anag.) + N