Representative period with book (5)
I believe the answer is:
agent
'representative' is the definition.
(agent is a kind of representative)
'period with book' is the wordplay.
'period' becomes 'age' (age is a kind of period).
'with' says to put letters next to each other.
'book' becomes 'nt' (New Testament).
'age'+'nt'='AGENT'
(Other definitions for agent that I've seen before include "Dealer" , "Double-zero-seven, for example" , "Representative - means" , "Sales rep" , "One acting on behalf of another" .)