Frightening word initially kept in reserve (4)
I believe the answer is:
book
'reserve' is the definition.
(to book something is to reserve it)
'frightening word initially kept' is the wordplay.
'frightening word' becomes 'boo' (word that may be said to surprise someone).
'initially' indicates taking the first letters.
The first letter of 'kept' is 'k'.
'boo'+'k'='BOOK'
'in' is the link.
(Other definitions for book that I've seen before include "Written work; reserve" , "an item from the library" , "Reserve - bets taken on a race" , "Reserve - publication" , "Reserve - volume" .)