1 and 20 From 15 down to what one might put on the table, this is very complete. (4,5,2,6)

 6 One version of early Irish writing. (Go back to an early Biblical character to find it.) (5)

 9 They shouldn’t properly be business trips. (7)

10, 11 and 15 down Good advice with a story of a leader of Israel during Genesis, perhaps. (5,2,3,9)

12 Class might be wearing, but not so much as before. (6)

13 Indian dance, colloquially, with one. (4)

15 What might be found in the ring (in spectator sport) and in the middle (in active game) might be one jump ahead of you. (8)

16 Could it be more pure, in a sense, and still wither the possible outgrowth? (6)

18 See 25

20 See 1 across

23 Bill, you might take more than one during the 12! (4)

24 You might look for logs in them, or perhaps more finished wood for furniture. (6)

25 and 18 Make a sort of joke in kindly (though biting) fashion. (9)

28 A form of element is nothing to drink to excess! (7)

29 Probably not boarding if you’re just doing this. (7)

30 23 found on the bank might be this on back. (5)

31 See 1 down

DOWN

 1  and 31 A demand for the appearance of only a middling facade? (5,3,6)

 2 Nothing at breakfast, for example, but this? (7)

 3 Not where the IRS keeps its "Married and Filing Jointly" returns, as you might follow. (6,4)

 4 One might dictate what your taxes will be. (8)

 5 and 19 A questionable stage line? (2,2,2,3,2,2)

 6 One probably has to be indulgent with this. (4)

 7 Shakers probably have one, for the sake of charity. (7)

 8 She’s the boss in the family! (9)

14 Daisy-type apart, it might be a supermarket. (5,5)

15 See 10 across

17 Caught, familiarly, with a ring around the neck. (8)

19 See 5

21 It’s probably only an impression, but they say it’s one way to bestow a kiss. (7)

22 Is whipped cream a thing to take a shot with? (6)

26 A very long reign broken in Africa? (5)

27 A couple of movies were this yesterday, and free. (4)