A ruse thus set out in book (9)
I believe the answer is:
thesaurus
'book' is the definition.
(thesaurus is a kind of book)
'a ruse thus set out' is the wordplay.
'set out' is an anagram indicator.
'a'+'ruse'+'thus'='arusethus'
'arusethus' with letters rearranged gives 'THESAURUS'.
'in' is the link.
(Other definitions for thesaurus that I've seen before include "lists?" , "Wordy gathering" , "work we may refer to" , "Wordy book" , "Reference work" .)