Fairly gentle in the DA world, but some pretty tricky wordplay (15A specifically, and less so 5D). Not sure about my parse of 18D, I don't see enough indicators in there to flip IEL into LEI without putting the anagram indicator after the fodder, which just feels odd. 25A definitely justified its mid-clue question mark!

Across

1

Gen-Xer’s dad, Joey? (4,6)
BABY BOOMER — cryptic def. (young BOOMER, i.e. kangaroo)

6

(& 21-down) Manchego? Too bad! (4,6)
HARD CHEESE — double def.

9

Two flash companions making Gabba blazer? (10)
FLOODLIGHT — cryptic def. (flash FLOOD + flash LIGHT)

10

Extreme stand to limit Cuban veto, flying missile (4)
SCUD — SD (outside letters of stand) around CUban without (flying) ban (veto)

12

Old fridge (since gutted) I lug to head back into eatery (10,4)
COOLGARDIE SAFE — CAFE (eatery) containing reversal (back) of SE (since without middle letters) I DRAG (lug) LOO (head)

14

Stout from Kentucky mended hearts (6)
ROTUND — middle letters of fROm kenTUcky meNDed

15

Detective sees wildebeest by dam, improving? (6,1.1.)
MAGNUM P.I. — MA (dam) GNU (wildebeest) anagram (roving) of IMP

17

Evoke crim concealing a wayward influence (4,4)
CALL UPON — CON (crim) containing A reversal (wayward) of PULL (influence)

19

Studio network packing pair of socks? (3-3)
ONE-TWO — hidden in studiO NETWOrk

22

Kitchen coinage used for culinary conclusion,… (5,2,3,4)
ICING ON THE CAKE — anagram of KITCHEN COINAGE

24

…where most of fast food repelled slob (4)
GRUB — reversal (repelled) of most of BURGer (fast food)

25

African hunter grasps anaemic (?) survivor’s insight (4,6)
LIFE LESSON — LION (African hunter) containing FE-LESS (anaemic)

26

Caught handful that was close! (4)
PHEW — homophone of FEW (handful)

27

Magi's pong stuffed stuff (5,5)
THREE KINGS — THINGS (stuff) containing REEK (pong)

Down

1

Polish nut (4)
BUFF — double def.

2

Lavish spree in flat? (7)
BLOWOUT — double def.

3

Wag glad you began suggesting cold feet or sweet tooth? (4,8)
BODY LANGUAGE — anagram (wag) of GLAD YOU BEGAN

4

Luke's mentor bloodless below the belt? (3-3)
OBI-WAN — OBI (belt) WAN (bloodless)

5

Chiefly, insect wiped out to erect smooth, spiky layers (8)
ECHIDNAS — anagram (out) of CHIEfly without fly (insect), reversal (to erect) of SAND (smooth)

7

Extol cooler insurance case (7)
ACCLAIM — AC (cooler) CLAIM (insurance case)

8

Instrument played Ring Ring after ghastly dirge (10)
DIDGERIDOO — DID (played) anagram (ghastly) of DIRGE, O O (ring ring)

11

Heard she rents rasp in northern town (7,5)
TENNANT CREEK — homophone of TENANT (she rents) CREAK (rasp)

13

Laughing at task of inflight omelette chef? (8,2)
CRACKING UP — cryptic def.

16

Fairly new party's backing new housing (8)
YOUNGISH — Y (last letter of party) anagram (new) of HOUSING

18

Sport field off-limits, becoming fixed (7)
LEISURE — anagram (becoming?) fIELd (without outside letters), SURE (fixed)

20

Shelters suckers (5,2)
TAKES IN — double def.

23

Strain our treat? (4)
ONUS — ON US (our treat)