Person who investigated difficult situations involving money (6)
I believe the answer is:
holmes
'person who investigated' is the definition.
(Sherlock Holmes)
'difficult situations involving money' is the wordplay.
'difficult situations' becomes 'holes' (hole can mean a tricky or difficult situation).
'involving' indicates putting letters inside.
'money' becomes 'm' (abbreviation in economics).
'holes' placed around 'm' is 'HOLMES'.
(Other definitions for holmes that I've seen before include "Watson's idol" , "One good at interpreting clues" , "Conan Doyle's creation" , "See 3" , "Famous detective" .)