Night before book incident (5)
I believe the answer is:
event
'incident' is the definition.
(thesaurus)
'night before book' is the wordplay.
'night before' becomes 'eve' (I've seen this before).
'book' becomes 'nt' (I've seen this in other clues).
'eve'+'nt'='EVENT'
(Other definitions for event that I've seen before include "Affair" , "Happening, noteworthy perhaps" , "pole vault maybe" , "Something going on" , "Planned public or social occasion" .)