Most important when backing story books (7)
I believe the answer is:
salient
'most important' is the definition.
(I've seen this before)
'when backing story books' is the wordplay.
'when' becomes 'as'.
'backing' is a reversal indicator.
'story' becomes 'lie' (story can mean a fabrication).
'books' becomes 'nt' (New Testament).
'as' backwards is 'sa'.
'sa'+'lie'+'nt'='SALIENT'
(Other definitions for salient that I've seen before include "Leaping (heraldry)" , "Prominent, most noticeable" , "Conspicuous; pointing outwards" , "Prominent, striking" , "defence work" .)