Person who investigates pilfered money (7)
I believe the answer is:
pistole
'person who investigates pilfered money' is the definition.
The definition suggests a singular noun which matches the answer.
'person who' is the wordplay.
'person' becomes 'pi' (PI is an example).
'who' becomes 'stole' (I can't explain this - if you can you should believe this answer much more).
'pi'+'stole'='PISTOLE'
Can you help me to learn more?
(Other definitions for pistole that I've seen before include "Spanish gold coin" , "old gold coin" , "bit of gold" , "Old Spanish coin" .)