Bring in a book of great significance (9)
I believe the answer is:
important
'of great significance' is the definition.
(I've seen this before)
'bring in a book' is the wordplay.
'bring in' becomes 'import' (I've seen this before).
'book' becomes 'nt' (I've seen this in other clues**).
'import'+'a'+'nt'='IMPORTANT'
(Other definitions for important that I've seen before include "major" , "Crucial" , "influential" , "material" , "Eminent" .)