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...