6A and 42 (whose unchecked letters give IDEA) combine to suggest the title of a novel. Remaining unclued lights give the forenames (in one case a nickname) of six of its characters, whose surname (5) will appear diagonally in the completed grid and must be shaded. Elsewhere, ignore two accents.
 

Across

1    Interceder tried maxi getting dressed (9)
9    Sloth-like nursemaids? (4)
11    Decorum in good session with spirits (10)
12    Terrier requiring food (4)
14    Drink Charles imbibed as dyspepsia cure (6)
18    Gross aristo snubbed knight (4)
20    Kay sat fiddling with classy kimonos (7)
21    Decay infecting maple pedestal (7)
23    Content of burrito cooked in iron dish (7)
24    Painter is saucy in conversation (5)
25    Fool about with English poem (5)
27    Lottery prize is this Scot: Frank! (7, hyphened)
30    Poles crew for Charon? (7)
32    Route first-born talked about with Oscar’s butler (7, hyphened)
36    Chief knave with cur’s heart (4)
38    Malign lady pockets sixth letter(6)
39    Go slow inside city (4)
40    European celebrity still painted by artist (10)
41    Wavy Navy in uniform dyed regularly (4)
43    Empresses freely assist art (9)
 

Down

1    Priestess in pulpit after mass (5)
2    Bit of durra – spot of cereal (5)
3    Most gifted alto made joyous (6)
4    Old viol famous rioter chucks about (5)
5    Decorator wearing coat (7)
6    Aircraft designer from hythe in Kelso (7)
7    Woman is engaging court and makes law (6)
10    Nasty Peter, icier and very unsympathetic (11)
13    Bag carried by little lass (7)
15    Most like lute (easily transported in case of concert) (8)
16    Crop lauded and valued again (11)
22    Actually excellent tailored coat (7, two words)
26    Maybe red-bodied and blue-horned bats meander (7)
28    Compounds of actinium turned dull bluish-grey (7)
29    Fanny Adams entering canon’s house (6)
31    Pancake contains caviar, perhaps it’s tossed (6)
34    Workers over in cattle farms? (5)
35    Prosecutor pitched up in courts (5)