ACROSS

 1  and 6 What you get when you spill your glass while sitting things out? (It may have to be treated.) (5,2,3,4)

10  Describing the acquaintance you barely know, but who appears to be a bit tired? (7)

11  If you do it, you may be sorry! (7)

12  Something on the list which may be a mite different. (4)

13  An unimposing figure of a man, but namely a governor of New York with something to catch, and returning to a Parisian. (10)

15  and 16 You shouldn't expect to have one follow your behavior if you do. (3,1,3,7)

17  Proving the communists are about to come first, as one does to watch every day if you have the right kind of a ticker. (7)

20  A bit confused, Lear suffers a setback by the father of Odysseus. (7)

22  If one insists "Show me!" he or she may be one. (10)

23  If she's a Londoner, she could certainly keep busy around the house, and slightly burn the fish! (4)

25  In earnest, but slightly confused, you have the closest thing to it. (7)

26  Not a slight change when going to a place in South America, as they say–but this is in Canada. (7)

27  and 28 Having this, you can't be afraid to do most jobs! (4-10)

DOWN

 1  With such as clarinets and saxophones, you shouldn't have to, as you must with old turntables. (4,11)

 2  Its diet is certainly different, though least messy. (7)

 3  One with a forked tongue is coming up here, when traveling by it. (4)

 4  Grading letters from a very stingy person. (7)

 5  A foot soldier of old Greece gets a head start, and could be polite finally. (7)

 7  In Paris no sort of post could be expected to go on indefinitely. (7)

 8  A classic example of tightness. (8,7)

 9  Gets in, having no romantic companion, but flooded by the response. (9)

14  A group of players is about finished, and left out cold! (9)

18  Lived at one time to go over the water–"Here's to your health!" (7)

19  Part of the fugue could be a sort, and in Paris it comes to a junction inside it. (7)

20  With a heavy weight away from it, it's at least a start! (4-3)

21  The sort found sort of near a city. (7)

24  What may go in the collar of a horse is a kind of poker. (4)