Cover coming before book? (7)
I believe the answer is:
pretext
'cover' is the definition.
('pretext' can be a synonym of 'cover')
'coming before book?' is the wordplay.
'coming before' becomes 'pre'.
'book?' becomes 'text' (text is a kind of book).
'pre'+'text'='PRETEXT'
(Other definitions for pretext that I've seen before include "Fictitious reason in order to conceal the real one" , "Excuse for an action" , "Excuse, ruse" , "Fictitious excuse" , "False reason as justification for an action" .)