Vehicle needing day to book (4)
I believe the answer is:
card
'book' is the definition.
(card is a kind of book)
'vehicle needing day' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'needing' says to put letters next to each other.
'day' becomes 'd'.
'car'+'d'='CARD'
'to' acts as a link.
(Other definitions for card that I've seen before include "showing heart, say" , "One taken in hand" , "Bit of a wag - joker, perhaps" , "king or queen, for example" , "Christmas greeting" .)