All pretty fair, but some clumsy cluing here and there. Lots of superfluous words, and 6D in particular is a bit strange. "Plan" as an anagram indicator is new to me, too.

Across

1

Justified rage is tougher (9)
RIGHTEOUS — anagram (rage) of IS TOUGHER

6

Puts one through torture after loads of wine (5)
RACKS — double def.

9

Allegro updated to show band (5)
GROUP — hidden in alleGRO UPdated

10

Document holder for high speed working party (9)
CLIPBOARD — CLIP (high speed) BOARD (working party)

11

Talk first, then judge like a judge (10)
MAGISTRATE — MAG (talk) IST (first) RATE (judge)

12

Our genes are short of one's longing (4)
URGE — oUR GEnes without ones

14

Stiff laces he knotted on foot race (12)
STEEPLECHASE — STEEP (stiff), anagram (knotted) of LACES HE

18

Mars her pet dental plan (3,3,6)
THE RED PLANET — anagram (plan) of HER PET DENTAL

21

Rank outsiders in tiebreaker (4)
TIER — outside letters of TIEbreakeR

22

After guy was cut short, is anyone cross-dressing? (10)
MAYONNAISE — MAn (guy cut short), anagram (cross) of IS ANYONE

25

Hard macaroni messed with mouth organ (9)
HARMONICA — H, anagram (messed) of MACARONI

26

Scoop up edging on external collar (5)
LAPEL — LAP (scoop up), EL (outside letters of external)

27

Whine about the origin of sprung rhythm (5)
PULSE — PULE (whine) around S (first letter of sprung)

28

Environmental order kept oriental tea warmer staunch (9)
ECOSYSTEM — E (oriental) COSY (tea warmer) STEM (staunch)

Down

1

Frost covering, say, the governing state (6)
REGIME — RIME (frost) outside EG (say)

2

Staggering after horse takes part in drunken orgy (6)
GROGGY — GG (horse) inside anagram (drunken) of ORGY

3

Breed dog for print worker (10)
TYPESETTER — TYPE (breed) SETTER (dog)

4

O for NATO's initiative on outer space vehicle! (5)
OSCAR — OS (first letters of outer space), CAR (vehicle)

5

Can Black singer, in Spooner's way, spark? (9)
SCINTILLA — Spoonerism of TIN CILLA

6

Invest in outfit from the Boer War (4)
ROBE — anagram (war) of BOER

7

Group leader sat half an inch above pilot (8)
CHAIRMAN — CH (half of inch) above AIRMAN (pilot)

8

Driven to despair by windy dead ends (8)
SADDENED — anagram (windy) of DEAD ENDS

13

Script adapted from Spencer poem (10)
SCREENPLAY — anagram (adapted) of SPENCER, LAY (poem)

15

Put weight on heap, somehow keeping mass on scale (9)
EMPHASIZE — anagram (somehow) of HEAP with M (mass) on SIZE (scale)

16

Sell down the river to make up the rent? (6,2)
STITCH UP — cryptic def.

17

Brother trapped in maize, say, of the mind (8)
CEREBRAL — BR (brother) inside CEREAL (maize, say)

19

Lease for gathering urchin and shellfish (6)
LIMPET — LET (lease) around IMP (urchin)

20

Put to sleep with sheep's endless confusion (6)
BEDLAM — BED (put to sleep), LAMb (sheep, endless)

23

Kiwi council gets zero on both sides of the ticket (5)
OTAGO — O (zero) on either side of TAG (ticket)

24

Warning only a small number heard (4)
FORE — homophone (heard) of FOUR (only a smally number)