Eh, strong "meh" from me today. Really clumsy cluing and weird mechanisms. 1A felt unfair, 6A has a useless word, 22A seems completely wrong to use "Mounte" instead of "Mountie". In general, nothing felt particularly precise and snappy.

Across

1

Hanging on, so close to invasion (9)
ATTACHING — ATTACkING (invasion, but not quite)

6

In a halo of brilliance, indifferent (5)
ALOOF — hidden in hALO OF

9

Little animals keep one viceroy in food (7)
PIGGIES — I (one) GG (viceroy) in PIES (food)

10

Look casually first at earth's extremes of positions (7)
LOCATES — LO (look) C (first letter of casually) AT ES (outside letters of earth's)

11

In the same place as first in command (4)
IBID — I (first letter of in) BID (command)

12

Beth oils me strangely - how joyful (10)
BLITHESOME — anagram (strangely) of BETH OILS ME

14

Problem-setting routine's regular occurrence heard after a long time (9)
ALGORITHM — A L (long) GO (time) homophone (heard) of RHYTHM (regular occurrence)

16

One man's woman (5)
IRENE — I (one) RENE (man)

18

Cultivation of some lentil thicket (5)
TILTH — hidden in lenTIL THicket

20

Downgrades rhenium representatives (9)
RELEGATES — RE (rhenium) LEGATES (representatives)

22

Buffoon in Canadian copper margin, say (10)
MOUNTEBANK — MOUNTE (Canadian copper) BANK (margin, say)

24

Note Irish trade show (4)
FAIR — FA (note) IR (Irish)

26

Mule race (7)
SLIPPER — double def.

27

Light toast gives skin damage (7)
SUNBURN — SUN (light) BURN (toast)

28

The Italian signature, first magic symbol (5)
SIGIL — SIG (signature) IL (the Italian)

29

These lads otherwise imperishable (9)
DEATHLESS — anagram (otherwise) of THESE LADS

Down

1

Hopeful a very quiet Liberal is not finished and is not able (9)
APPLICANT — A PP (very quiet) L (liberal) I (is without last letter) CAN'T (is not able)

2

Putting out of the game for following (7)
TAGGING — double def.

3

Rebuked church identity (4)
CHID — CH (church) ID (identity)

4

It isolates an old Roman tenement on a hill (9)
INSULATOR — INSULA (an old Roman tenement) TOR (hill)

5

Key to government apartment (1,4)
G FLAT — G (government) FLAT (apartment)

6

Dreaming about low pain, call on one (10)
ALCHERINGA — L (low) in ACHE (pain) RING (call) A (one)

7

Poll better than open book in heavenly centre (7)
OUTVOTE — OUT (open) OT (book) in VE (centre letters of heavenly)

8

Charge around ship with a heraldic device (5)
FESSE — FEE (charge) around SS (ship)

13

Revolutionary in skill, play travesty of the first form (10)
ARCHETYPAL — CHE (revolutionary) inside ART (skill) anagram (travesty) of PLAY

15

Terrible loss of memory about the Spanish islands (9)
MELANESIA — anagram (terrible) of AMNESIA (loss of memory) around EL (the Spanish)

17

Measures outside of odd sphere in east and holds as sacred (9)
ENSHRINES — ENS (measures) outside SHR (odd letters of sphere) IN E (east)

19

Celebrating dual comeback in grand opening (7)
LAUDING — reversal (comeback) of DUAL, IN G (first letter of grand)

21

Defame foolish curate about a penny (7)
TRADUCE — anagram (foolish) of CURATE around D (penny)

22

Enquire in writing for covers (5)
MASKS — ASK (enquire) in MS (writing)

23

Left exposed academic blushing (5)
BARED — BA (academic) RED (blushing)

25

A small measure of chain put by inside (4)
INCH — IN (inside) CH (chain)