1 and 12 The Detroit Lions, figuratively, or what they might show. (12,5)

10 Double zero thrown in a sign of subtraction might be portentous. (7)

11 On the level, is part of sitting up late a usual high spot? (7)

12 See 1

13 A form of transportation before carts designed for absolute rulers. (9)

14 Aerie high above the gentle sea’s possible turbulence. (6,4)

16 Natural power, alternatively a figurative trace. (4)

18 In short, it might herald the fall of a clan. (4)

20 What one associates with him is really a place in Ireland. (6,4)

23 On the podium, or on the board? (9)

25 You might so insinuate it’s simply decapitated. (5)

26 I had what looks like ten to shake, silly! (7)

27 You might mark him zero in biology for not recognizing it. (7)

28 Burns might be so described, with an M.A. or M.S., possibly. (6,6)

 2 and 19 Not what hungry gate-crashers might be doing, but they’re certainly offensive groups. (7,7)

 3 When an animal comes up to a different sort of creature, shoot this way, possibly. (8)

 4 In a different case we get up dirty, and get kept this way. (9)

 5 Grist for the mill, as one might say. (5)

 6 With a parvenu, ancestors are bound to show a slight difference. (6)

 7 Where you get to pull a fish up? (There are such islands.) (7)

 8 Is the tune squashed into a few bars and perhaps put into a cylinder? (10,3)

 9 Do they involve the patter of feet more or less matched and regularly presented where such might be heard? (7,6)

15 Bad grades get corrected to show how one behaved if not a steady course follower. (9)

17 Such heartless advertising implies making a few suggestions. (8)

19 See 2

21 Go off quietly? Anything but! (7)

22 Face-tickling shows a sour expression. (6)

24 Where one might find ready-made food for the South American. (5)