I am stepping out now, have a function to attend, will be updating the blog with the explanations later in the day. Till then, happy solving.

Across

1 Become less tense (4,3)

5 A number of people don't start, with accountant and politician, to live in tent (6) ENCAMP [MEN(A number of people)-M(dont start) + CA(accountant) + MP(politician)] - I guess would be "CLUE OF THE DAY"

9 Picture of one performer (5) IMAGE [I(One) + MAGE(Performer??)]

10 Get involved with Evert and nine in a whirl (9) INTERVENE [Anag of EVERT & NINE]

11 Bring to court everyone linked with mischievous boy (7)

12 Bolster supplies of shellfish (7) LOBSTER [Anag of BOLSTER]

13 That, that, that, that... (5)

14 Pasta made in ribbons (9)

16 Gleans ten are involved in snags (9) ENTANGLES [Anag of GLEANS + TEN]

19 Nag a mammal (5)

21 Bringing up excellent son in arena (7)

23 Greek character against wine (7)

24 Formidable task that high-rise builders can fulfil (4,5) TALL ORDER [Direct Clue??]

25 Bad moral is in one's mouth (5) MOLAR [Anag of MORAL]

26 It supplies blood (6) ARTERY [Direct Clue]

27 No tears shed for politician (7) SENATOR [Anag of NO TEARS]

Down

1 What a Commission of Inquiry is expected to do (6,3,5)

2 Cut and blow-dry may follow it (7)

3 Or weave around to intimidate (7) OVERAWE [Anag of OR WEAVE]

4 Female just atrocious (9)

5 Crack up! (5)

6 Cartoon character, not ill, holds a bone of Arctic deer (7)

7 Scholar, Oriental, nearly authoritarian with eminent music composers, for example (7) MAESTRO?? [Direct Clue??]

8 As used for a backward glance while driving (4-4,6) REAR-VIEW MIRROR [Direct Clue]

15 Tracts Ed's recast may be seen sometimes on TV monitor (4,5)

17 Not one to be taken seriously (7)

18 Racket around a sacred chant is offensive (7) NOISOME [Container Clue - noisOMe]

19 Snap him! Good enough for a sailor (7) SHIPMAN [Anag of SNAP HIM]

20 Curl in the crown (7)

22 Being divine, saint and I leave stodgily, vexed (5) GODLY [ST(Saint) + I leave STODGILY to get GODLY]