Get older part of bible for spy (5)
I believe the answer is:
agent
'for spy' is the definition.
(secret agent)
'get older part of bible' is the wordplay.
'get older' becomes 'age' (synonyms).
'part of bible' becomes 'nt' (New Testament abbreviation).
'age'+'nt'='AGENT'
(Other definitions for agent that I've seen before include "Negotiator" , "One acting on behalf of another" , "Delegate" , "Business manager" , "One does business on behalf of another" .)