Expert detective captivated by old money (6)
I believe the answer is:
pundit
'expert' is the definition.
(I know that pundit is a type of expert)
'detective captivated by old money' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'captivated by' means one lot of letters goes inside another.
'old money' becomes 'punt' (I can't justify this - if you can you should believe this answer much more).
'di' put into 'punt' is 'PUNDIT'.
Can you help me to learn more?
(Other definitions for pundit that I've seen before include "Subject expert" , "Expert (sometimes self-appointed)" , "Person who is an authority on subject" , "Public expert" , "Maestro" .)