Solving Time:  68 minutes

Probably the toughest Guardian crossword I have encountered in a very long time. 
I made a good start with the big anagram but it just went backwards with unfamiliar words and cleverly hidden allusions within the clues. 

Some really great clues with 21 across being a real doozie.  22 across likewise was  extremely clever.

Many thanks to Pasquale for a ferocious battle that brought about a major sigh of relief when I wrote in my last entry.
ACROSS   * = Anagram   (CD) = Cryptic Definition   (R) = Reversal

1 see 6 down
4 SPADIX  SPA + DIX (How they would say “ten” in the L. Gardens!)
9 ONER  This really derailed me.  I entered ICON as being a hidden answer.  Eventually, after convincing myself as to the right actress at 2 and the word at 1 down, I twigged this relates to “The Three R’s” - some one just good at Maths would be a ONE R!  Bit too clever that clue!
10 PLASTIC ART I presume the definition is “What may involve modelling ceramics”
   and the wordplay is “tar?”  ie  plastic (fake) ART.  I am happy to be corrected if I have misjudged this.
11 PLAY UP  LAY (to station) inside PUP
12 SHREWISH R in SHE + WISH
13 CASTOR OIL NANCY  =  Astor inside COIL (To wind)
15 ARMS  A Root Mean Square (RMS)
16 BRIG  Two definitions 
17 BLOCKHEAD BLOCK + HEAD
21 BOOKSHOP The She in this context is Rider Haggard’s novel called “She”!  Outrageous but great fun!
22 SHLOCK  SHYLOCK (Unfair dealer) minus Y
24 COLLATERAL L + LATER inside COAL
25 WHIT  Two definitions
26 TISANE  TIS = It’s + A + N + E (Drug)
27 ODDS ON  ODD + SON
0
0DOWN
1 MANILLA  ILL inside MANA (Maori form of power)
2 TERRY   Another clever clue TERRIFY minus IF (Kipling’s lines) 
3 ON PAPER Two defs
5 PATERA  PATER + A
6,23, 1AC  DUCKWORTH LEWIS METHOD THE LORD’S WICKET MUD HOW*  Nice appropriate anagram!
7 XEROSIS  X + EROS + IS
8 MANSFIELD PARK MAN + SPARK with field INSIDE
14 TRISKELES RISK inside STEEL*  Another new word on me
16 BLOWOUT  LOW (Mean) + O (Egg) inside BUT (Bar)
18 CASTLED  CAST + LED  A ’special move’ in chess
19 AUCTION  This looks like a cryptic def relating to bidding (Auction) in bridge.  Thanks to BeerMagnet: Caution (Take care) with C (Clubs) shifted down a little.

20 PHOTON  PHOTO + N