Six unclued lights are 18 Down from the same source (five are entitled to appear; the sixth appears regularly). Six cells initially contain between two and six clashing letters; these must be resolved to display thematic 18. Elsewhere, ignore three accents and two apostrophes.
 

Across

 
10    A Republican politician maintains British dealt badly with defence material (12, hyphened)
12    Marsh tree (5)
13    In speech, lifts poet’s handles (7)
15    Awkwardly got up on camel bred to travel (9)
16    Crack plus heroin – a drug (6)
17    Protecting money, American wealthy mostly lacking heart? (8)
21    With musical backing, orator’s clipped style (8)
23    A test disheartened armed rank and file (7, two words)
26    Conservationists drilling rock pilaster (4)
28    Automaton reversing sledge, ditching goods for Norway (7)
29    Queen runs into trouble – cash reduced for carriages (8)
34    Cook’s crumby jar contains alcohol, not weak (8)
35    Draw one away from instrument (4)
38    Return of US novelist filled with rage — he succeeds (7)
39    Repealed requirement to carry gun (7)
40    Haystack’s support in street spoils atmosphere (12, two words)
41    Light grey seaweed on beach (6)
42    Nucleus’s density varying en route (8)
 

Down

 
1    Worker breaks into smoother port (9)
2    No druggy regularly refused binge (4)
3    Making fast work stopping cartel (6)
4    Nearly cut and mend Welsh dresser (7)
5    They fence in 250 animals — European one’s penned (8)
6    As some consonants following Anglo-Latin (7)
7    Stop Conservative with wealth (5)
8    Look to prevent reporter’s unbroken peeping here? (10, hyphened)
9    Bit of grass is also for plants (6)
11    10 god of war with time for marsh plant (9, hyphened)
14    Inferring siren may be doing it, topless (7)
19    Is a bonnet perhaps what penitents wore? (9)
20    American capital ready to acquire property (7)
22    Blush more than extravagant fool, endlessly sent up (9)
24    Philosophical term lunatic learns in early English (8, two words)
27    Terminal Roman poem (7)
30    After Mass, notice little room for incense (6)
31    Nobleman’s dodgy motive, ingesting cocaine (7)
32    Ex-Man U defender blocking Ronaldo’s header (5)
 
A first prize of £30 for the first correct solution opened on 19 May. There are two runners-up prizes of £20. (UK solvers can choose to receive the latest edition of the Chambers Dictionary instead of cash— ring the word ‘Dictionary’.) Entries to: Crossword 2160, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.