Rep books after a period (5)
I believe the answer is:
agent
'rep' is the definition.
(I've seen this in another clue)
'books after a period' is the wordplay.
'books' becomes 'NT' (New Testament).
'after' says to put letters next to each other.
'a period' becomes 'age' (age is a kind of period).
'nt' after 'age' is 'AGENT'.
(Other definitions for agent that I've seen before include "Catalyst" , "Middleman" , "One's representative" , "Spy; service provider" , "Substance that exerts some force or effect" .)