Crossword Genius

Person who investigates pilfered money (7)

Ross

I believe the answer is:

pistole

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'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" .)

I've seen this clue in The Guardian.
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