Across

1 There is nothing ordinary in such cinema technique (7,7) SPECIAL EFFECTS [Direct Clue]

8 Score less than a hundred (6) EIGHTY [CVasi : score is twenty; 20 less than 100 is the answer.]

9 Man of religion who advises on behaviour (8) PREACHER [One who preaches]

11 Killer of Boston, an alien holding student (9) STRANGLER [CVasi : L (learner, student) in stranger; ref to the Boston strangler]

12 Eastern ruler is making the supply last (5) EKING [E+KING]

13 Getting close to earning a change (7) NEARING [Anag of EARNING, change is the anag indicator]

15 Would a C.O. mind becoming itinerant? (7) NOMADIC [Anag of A CO MIND]

17 Shade for a representative in some degree (7) MAGENTA[M AGENT A - MA is a degree and reperesentative, AGENT in MA gives MAGENTA]

19 Beneficiary under a policy (7) ASSURED [Direct Clue, Initially thought it was ASSURER]

21 Ring back around one for flower (5) LILAC [To ring is CALL and around one is I, Anag of I + CALL gives LILAC]

23 Where one studied in Latin (4,5) ALMA MATER [Direct Clue]

25 Worn out by direct physical exercise, oddly (8) DECREPIT [CVasi : anag of direct PE (abbr for physical exercise)]

26 One of several English kings involved in an antiquated war dance (6) EDWARD [Hidden Clue - antiquatED WAR Dance]

27 With a dark glance, a dog ran, suggestive of intrigue (5-3-6) CLOAK-AND-DAGGER [Anag of DARK GLANCE DOG]

Down

1 Top cover — let it stand, boy! (7) STETSON [CVasi : stet - son; stet is a proofreader's instruction to the printer; when one scores out and then immediately has second thoughts, he underlines the struck out word with a broken line like this and writes STET on the margin -meaning let it stand - don't effect the striking out]

2 Avid, like the beaver? (5) EAGER [Direct Clue]

3 The purpose in stress, say (9) INTENTION [IN + TENTION (sounds like TENSION)]

4 Serve as a herald with a warning and flee (7) P[]

5 Small latitude in Eastern Europe to fill with exhilaration (5) ELATE [LAT in EE - E LAT E]

6 Revenue official who had trails altered (9) TAHSILDAR [Anag of HAD TRAILS]

7 Cheroot thrown up after little time — appalling (6) TRAGIC [Cheroot is CIGAR and time is T, Anag of CIGAR + T gives TRAGIC]

10 A bullet for the mollusc (4) SLUG [CVasi : two def; slug is a bullet also oyster/snail/mollusc/small insect]

14 Mutation of Gail, clean and like a celestial being (9) ANGELICAL [Anag of GAIL CLEAN]

16 Stupid creature, male, in means of mass communication — press, TV, etc. (4,5) MASS MEDIA [creature here is ASS male is M and means is media as in via media CVasi : Since MASS is in the answer word, a more careful clue-writer would avoid the use of the word in the clue sentence]

17 Disease a boy got in a month (6) MALADY [MA lad Y]

18 Aibara caught mixing a kind of coffee (7) ARABICA [Anag of AIBARA + C (caught)]

19 The culminating point for an expert swallowing a bit of marjoram (4) ACME [Culminating point is the definition, expert is ACE and bit of majoram is M, so AC m E]

20 One who ridicules notes on proviso (7) DERIDER [CVasi : d e - notes, musical noes; proviso -rider (remember geometry or legal deeds]

22 Each learner turns into a disciple in India (5) CHELA [CVasi : anag of each L (leaner, student)]

24 Strum the guitar perhaps (5) TWANG [Direct Clue??]