3 letters:
ack, akc, bak, cac, dac, dak, fac, lac, lak, mac, mak, pac, sac, tac, yak, zak

4 letters:
back, flak, hack, jack, lack, mack, pack, rack, sack, tack, wack, yack

5 letters:
aback, anzac, black, brack, clack, crack, eniac, flack, gopac, ileac, knack, kodak, lilac, quack, shack, slack, smack, snack, stack, whack, wrack

6 letters:
anorak, attack, balzac, calpac, chirac, claque, cognac, compaq, hijack, ipecac, kodiak, maniac, plaque, prozac, ricrac, sacque, tarmac, tictac, unpack, zodiac

7 letters:
almanac, buyback, cardiac, carjack, cutback, dieback, elegiac, fatback, finback, gum-lac, hatrack, hogback, hopsack, jacques, outback, payback, pontiac, ransack, redback, setback, shellac, skyjack, tieback, tintack

8 letters:
ammoniac, amnesiac, backpack, bareback, blowback, bootjack, brainiac, cadillac, callback, clawback, coatrack, comeback, creashak, demoniac, drawback, etodolac, fallback, fastback, feedback, fireback, flapjack, fullback, gimcrack, grayback, gripsack, halfback, hardback, haystack, highjack, holdback, humpback, kickback, knapsack, mossback, packsack, playback, pullback, rollback, roorback, rucksack, simoniac, six-pack, skipjack, slapjack, snowpack, softback, sulindac, swayback, tailback, talkback, ticktack, wingback, woolpack

9 letters:
amberjack, antiblack, applejack, backtrack, barmbrack, blackjack, bootblack, bushwhack, call-back, cheapjack, crookback, crossjack, flashback, greenback, gunnysack, hatchback, hitchrack, horseback, huckaback, hunchback, ink-black, insomniac, jet-black, laid-back, lampblack, leaseback, merrimack, paperback, paranoiac, pickaback, piggyback, quillback, racetrack, razorback, shoeblack, sidetrack, slingback, sweptback, thornback, throwback, thumbtack, toastrack, touchback, wisecrack

10 letters:
ahorseback, blue-black, canvasback, cheap-jack, coal-black, cornerback, cul-de-sac, hackmatack, half-track, inky-black, knickknack, lumberjack, monomaniac, natterjack, pig-a-back, pyromaniac, saddleback, silverback, smokestack, soundtrack, supplejack, switchback, turtleback

11 letters:
biofeedback, click-clack, crackerjack, culs-de-sac, diamondback, dipsomaniac, hollow-back, ladder-back, leatherback, leatherjack, lobsterback, pericardiac, pitch-black, prickleback, quarterback, steeplejack, stickleback

12 letters:
back-to-back, bibliomaniac, bladderwrack, chimneystack, megalomaniac, nymphomaniac, off-the-rack

13 letters:
counterattack, high-low-jack

14 letters:
clickety-clack