9  ((Fourth of 16 + fourth of 8 + fourth of 4 + sixth of 12) by sixth of 18; ... or 60% of ... (5)

10 Oral examinations that prospective doctors have to submit to? (4,5)

11 Annoying a clerical person having oomph at one? No good ... (10)

12 Follower of Yankee, sometimes follows Bravo to convey appreciation of a job well done (4)

13 Number always used in writing heights and weights (5)

15 Styes and unsightly things (8)

17 Oracle is confused by a couple of females ... Go away! (5,3)

19 Check proper use of checks in America? (5)

22 Half-a-dozen turned upside down at hour just before 10 (4)

23 E'en leg cuff may be treated to show brightness (10)

26 Exercise energetically after stupid girl leaves ... It produces gas! (9)

27 Torn denim banned from dinnertime. Can it be harmless? (5)

28 Smother holy person, called Uriah initially, after designated time (11)

 1  He may challenge you or accept your challenge (5)

 2  Protects a misrepresented witness (9)

 3  Fastens metal fasteners disturbing rest around 4 (6)

 4  Three score and ten? That's not odd in a pig-pen (7)

 5  First, in the beginning, add one-ninth of 27 and sixth of 12 (4)

 6  Queer lad is worried about front of car having a protective coating (9)

 7  Best riders have pace (6)

 8  Sons'or Daughters' problems? (6)

14 Sudden's bluff displays inexperience (9)

16 Hans travels with damaged grail to mythical place (7-2)

17 Dancing dance endlessly .. once again ... dance! (6)

18 - 17 + 2 x (7)

20 Score a couple of sixes and fours (6)

21 Office furniture (6)

24 Brother's daughter's initial exam's in French city (5)

25 Person who changes colours paints red around end of hallway (4)