According to Brewer, a five-word phrase was used to describe a creature, but applies also to a three-word fictional character. The unclued entries, one doing double duty, comprise the phrase, the creature, his victim, the character, and the author’s surnames.
 

Across

1    ‘Poacher’s instruments’ mean whisky (8, hyphened)
6    A jockey’s is shorter than a cricketer’s or swimmer’s (6)
11    Somehow I trust in operating system — as do we? (10)
13    Virginia leaves unusual reading in fits (5)
14    Used deep-worn crinkles (8, hyphened)
16    Book at bedtime essentially involves unknown author (5)
17    Most of lie covered by idiot in leaflet (7)
18    Wraps up conclusions, having followed content (7)
19    A constant alto, very soft, Miss Fitzgerald’s sung unaccompanied (9, two words)
22    European country, lacking in resort (5)
23    Folk from Dover seizing a crew? (6)
24    Palms like rotating where Palm Springs is? (6)
26    Opponent of revolutionary Spain (3)
30    Thank Duma for changing capital (9)
32    Loamy ground closes around one (7)
33    Rocket engine more like early science fiction? (7)
35    Eva, perhaps, finding radioactive element in bread (5)
36    Current divinity giving two meanings for Isis? (8, two words)
37    Double one: game shot (5)
38    Assessing the removal of inhabitants, reducing 100 by half (10)
39    Term for gassy substance without mass: hot air in the Highlands (6)
40    It’s polarised voter missing old film (8)
 

Down

2    Old river in old Colombia? (7)
3    Rabbit cries ‘Monsters’ (7)
5    Shire horse’s failing late monarch (4)
7    University getting in European sweet dish (8)
9    Allowed to study cycling pedal (7)
12    Faint star, without oxygen replacing uranium (5)
15    One smoking in Sicily before getting up (4)
20    Whose piety may be questioned in Islamabad? (4)
21    Support league with 12 in? (4)
22    Help forward player waiting to play wait (8)
25    One lady almost captures Japanese island — this one? (7, two words)
27    Pistol killing German it set free (7)
28    Scorn once ridiculous seeding (7)
29    Girl interrupting couple’s meeting (5)
31    Graduate amongst present heads (6)
 
A first prize of £30 for the first correct solution opened on 4 February. 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 2391, The Spectator, 22 Old Queen Street, London SW1H 9HP. Please allow six weeks for prize delivery.