One unclued light, a term for a boundary of an area whose name is formed by two unclued lights, is a 19 of five items (one of which consists of two words, and one of which is hyphened) reading clockwise in the perimeter. Letters in corner squares and those adjacent to them could make CLAN GRANT END.
 

Across

11    Bird turning and not following man (5)
12    Camel? See it in Cairo on tour (4)
13    Rock in railroad carriage (5)
14    Be angry with burden, offloading old stock (7)
15    Report about standard of money, not very welcoming (10)
17    Smart books among those returned (5)
20    One variation of endless inactivity (8)
21    Radical in brilliant form ignoring boundaries (6)
23    Extra special advantage (4)
26    Refuse King Lear initially (5)
27    Engineer first to find fissures (5)
28    Stop wanting constant wealth (4)
31    Power tool adapted as a winch (8)
36    Poet using Celtic language, first to last (5)
38    Island with mostly low housing remaining at constant temperature (10)
39    Mineral oil troubled climber (7)
40    Fix and check hamper (5)
41    Grow spiritless following delay (4)
42    Tree sloth in activity after noon (5)
 

Down

2    Look, born into the world, worthy of affection (7)
3    Wine in room upset fool (10)
4    Veto in operation once refined (6)
5    Earth’s shape (5)
6    Boundaries only in abandoned atlas (8)
8    Ring at once about contest finally being drawn (5, two words)
10    Singer from provincial town (4)
16    Lifted cover, gold, of exceptional quality (5)
18    Bones starts to offer Spock some advice (4)
24    Increased energy author put into act (8)
25    Rubbish, not right for platform (4)
29    Being ascetic, keeping cold (7)
32    Check area inside aluminium drum (6)
33    Soldier in uniform after plunder, not first (5)
34    Part of leg, tense, in a raised position (5)
35    Cry of joy about publication in last stage of development (5)
37    Cope with ride on horseback (4)
 
A first prize of £30 for the first correct solution opened on 15 October. 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 2378, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.