Books to get on earlier vehicle (5)
I believe the answer is:
agent
'vehicle' is the definition.
(thesaurus)
'books to get on earlier' is the wordplay.
'books' becomes 'nt' (New Testament).
'to get on' becomes 'age' ('age' can be a synonym of 'get on').
'earlier' means one lot of letters go next to another.
'nt' put after 'age' is 'AGENT'.
(Other definitions for agent that I've seen before include "Salesman" , "Actor" , "that may be secret?" , "One acting on behalf of another" , "Person acting on behalf of another" .)