Brokers take a long time to get book (6)
I believe the answer is:
agents
'brokers' is the definition.
('agent' can be a synonym of 'broker')
'a long time to get book' is the wordplay.
'a long time' becomes 'ages' (ages can mean a very long time).
'to get' is an insertion indicator.
'book' becomes 'nt' (New Testament).
'ages' placed around 'nt' is 'AGENTS'.
'take' is the link.
(Other definitions for agents that I've seen before include "Sales reps" , "They act for actors" , "travelling salesmen" , "They do business on behalf of others" , "They act for others" .)