Bit of money I invested in fiddle (4)
I believe the answer is:
coin
'bit of money' is the definition.
'i invested in fiddle' is the wordplay.
'invested in' indicates putting letters inside.
'fiddle' becomes 'con'.
'i' going within 'con' is 'COIN'.
(Other definitions for coin that I've seen before include "Make up" , "Metal used as money" , "Mint" , "Invent (a new word)" , "Metallic money" .)