Thirteen unclued lights are of a kind (all singular, not plural) and confirmed in Chambers. 17 across gives the puzzle’s title (three words). Collins confirms 42A and the OED confirms 5D.

Across

1 What artists do wrong to extend (7, two words)
6 Sailor – another in sea is absorbed (7)
14 Maybe king rejected skill in hymn (6)
15 Clashed with officer and deserted shielded by number of Germans (8)
16 Returned reward for judge (4)
21 Advertiser tore off after concert (8)
23 Mathematician from Italy is easing off (6)
25 British engineer smuggles cases for weapons (8, two words)
27 Two thirds of Bible book in assembly (8)
30 Remark shifting energy of the mind (6)
34 Knocks back and beats ancient Greeks (8)
37 Unfortunate area, place besieged by Nazi (7)
40 B____ cheek! This is irregular (4)
41 Some potential fury probed by a French writer (8)
42 Among proletariat, richness of medics (6)
43 Frenchman drinking soft drinks (6)

Down

1 Run away from follower of aide? (6)
2 Return of the French fashion, wearing frilly fabric suddenly (9, three words)
3 Takes steps with negotiations leader’s left (5)
4 Struggled to get in two rounds in Spanish city (6)
5 Character caught by cuckoo bites antelope (8)
7 Sand’s dubious novel covers (7)
9 Mug’s lost one arm (4)
10 Atmospheric area, say, particle keeps touching (7, two words)
20 Journalist, I generalise, pens part of column (4)
22 Seabird first cut by seaman, one who’s self-sufficient (9)
24 Extremely eventful and unusual time period (4)
26 People who discuss hip-hop artists (8, two words)
29 Dodgy school without individual strength (7)
32 Fearful lout stopping still (6)
33 Parts of foot setter twice dipped in ocean (6)
35 German soldier in row retreating with hesitation (6)
36 Clare’s damaged ocular membrane (6)
38 Escape from resort, touring around (5)

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