Books to get on first vehicle (5)
I believe the answer is:
agent
'vehicle' is the definition.
(I've seen this before)
'books to get on' is the wordplay.
'books' becomes 'NT' (New Testament).
'to' says to put letters next to each other (I've seen this in other clues).
'get on' becomes 'age' (getting on means aging).
'nt' put after 'age' is 'AGENT'.
'first' acts as a link.
(Other definitions for agent that I've seen before include "Middleman" , "Person acting on behalf of another" , "One does business on behalf of another" , "He or she acts for actors" , "Representative - means" .)